Last one, so feel free to start complaining at me after this one. (I'm pretty sure I got all the math right, but if I didn't, go ahead and call me out.) Post 3 of 3:
Originality: 3.5 With a half-casting SI, I was expecting more chefs to use PrC-granted casting to fulfill the spellcasting requirement, but luckily for you, no one else actually did so. I may have been expecting it, but I was still surprised by you being the only one to actually do it, so that works out in your favor. [+0.25] Changeling Rogue makes an appearance in more Iron Chefs than not . . . yeah, it's an amazingly powerful level (one of the most character-defining single levels in the game, especially at level 1), but it seems like no SI is immune to someone throwing it in there. (I may have to ask one of our lorekeepers how many Changeling Rogues have actually shown up in Iron Chef, at least in the past year or so, because it seems like there's always at least one.) I'm not going to say that it's a bad choice, but it's the exact opposite of an original choice. [–0.25]
Savant, on the other hand, I didn't see coming. Perhaps I should have, since we already had Dragon Compendium open, but since it's usually so completely overshadowed by Factotum, I nearly forgot that it exists. For that matter, I didn't expect anyone to try to make a stealthy skill-primary character here—yeah, the SI grants a few random bonuses to skills, but with 2+INT skill points per level, the SI really isn't a class that you expect to live and die on its skills. So I really didn't expect anyone to pull out something like Assassin. I can give you some points for going with a character role I didn't really expect. [+0.5]
Power: 3.0 Your early levels see you as a fairly generic skillmonkey. Changeling Rogue is as useful as ever (like I said, it's one of the most character-defining single levels in the game), and you've got a couple dice of SA, so you can do some damage when you manage to hit (WF doesn't hit the field until 3, as I discuss in Elegance, and your BAB isn't great early on, but you'll be okay). Skill Assistance is a nice trick that I'm surprised we don't see more often. Overall, I think you've got one of the stronger starts out of this crop of entries, which is nice to see. [+0.25]
Your feats aren't really anything special. Three of them are, of course, eaten up entering the SI—that's unavoidable on some level, but you didn't really put any of them to extra use, so there's a large chunk of build resources that are just kind of sitting there. Deceptive Spell would be very thematically appropriate if you had any spells where you needed the visual effect to come from somewhere else, but since most of your spells are either personal buffs, touch attacks, or mind-affecting tricks with no real visual component, you don't really have a whole lot of use for Deceptive Spell . . . which means that it's nearly a wasted feat. I'm also not sure how much good Brew Potion is doing you. I definitely appreciate that you want to guarantee that you have access to potions to use with Polyhaemia, so I do understand why you took it, but you have very few spells known that can be brewed into potions. Remember that personal spells (or spells above 3rd level, for that matter) can't be made into potions, so you can't use a lot of your stuff like Vital Strike, True Strike, Swift Invisibility, Veil of Shadow, Toxic Tongue, and so on. Ebon Eyes would work, and Amorphous Form would work, and I'm honestly not sure whether or not Blade of Pain and Fear would work. So that puts you at either two or three spells that actually can be made into potions. Not really a shining example of an incredibly useful feat. Overall, I'm seeing more dead weight in your feat list than I really like to see. [–0.25]
I do think Staggering Strike is a good feat. For most of the time you have it, you've got 4d6 SA, which isn't going to break any damage records, but at least it's noticeable. You have enough swift-action defensive spells that I'm willing to believe that you might actually get away with making a melee sneak attack or two, and you might even have a way of debuffing a Fort save, so I can see you getting some use out of that. Making enemies unable to full attack you is a pretty good way of staying alive, so I like that. [+0.25]
Unfortunately, you didn't really pay your feat taxes. You've got maxed Hide and MS, but you don't have Darkstalker, which means that anything with Scent, Blindsense, Blindsight, or Tremorsense (and I don't need to tell you how dirt-common those are, do I?) can see you anyway. I absolutely hate that Darkstalker is a feat tax for a character archetype that doesn't necessarily have a lot of free feats anyway, but the fact remains, without Darkstalker, stealth tends to be a losing battle. (It also would have been nice to see HiPS in there somewhere, but I understand that getting HiPS is a nontrivial investment much of the time.) It's clear what the character is aiming to do, but it's not clear how well you'll succeed at that past the earliest levels with neither Darkstalker nor HiPS. (Swift Invisibility is a band-aid, but you don't have enough spell slots to rely on it all day.) [–0.25]
Elegance: 2.75 Bad news, mate. You don't qualify for Weapon Finesse. It requires BAB +1, which you don't have at level 1. Yes, that's incredibly dumb, and in a home game, I'd houserule that away before you could blink, but you're still not legal as written. You could be legal just by swapping around your 3rd level feat and your 1st level feat (even if that does make your first two levels a bit harder), so I won't give you a zero here or anything, but the fact remains, the build isn't legal. [–0.25] At the very least, your skills are all legal—no bonus for that (that's a baseline assumption, and you don't get a cookie for meeting baseline assumptions), but at least I don't have to penalize for that.
I'm not seeing any elegant ways of handling prereqs or of making them do double duty. The closest thing is using Assassin casting to qualify for having 2nd level spells, but that's not really anything groundbreaking, and it does mean that you get into the SI later than if you were a base casting class. (It might help you in UoSI by making it so that the SI didn't cause you to lose caster levels, but it's not so elegant that I'm really impressed by it, if that makes sense.) I guess it's nice to use Savant's open-ended bonus feat to get one of the SI's prereqs out of the way, and I do appreciate using Savant's skill list to get the weird mix of skills you need for Flux Adept out of the way, but I'm not seeing anything that really wowed me in that department, since the skill points and the feats still kind of got wasted (you got into the SI, and then you never did anything with them ever again). 
By the very strictest reading of the rules, you don't qualify for Detach, since technically you pick feats before you get new class features (PHB pg. 58-9), meaning that you technically don't have Regeneration until after you pick your 18th level feat. That said, I've never met a GM who's going to be quite that strict about things, so I'll mention it, but I won't penalize for it. Besides, it's funny. Sure, it's not strong, but it's funny.
Overall, I don't see a whole lot that's really painfully inelegant (aside from the unpleasant prereqs on the SI), but I don't see a whole lot that's especially elegant, either.
Use of Secret Ingredient: 3.0 Let's just go step by step. You have at least a few token ranks in everything Grace Through Will gives a bonus to—in a perfect world, I'd like to have seen more ranks in them, but with the crappy skill points you get from this class, that's not really possible, and at least I feel like you're making more of an effort on that front than many of your comrades. I'm torn on whether Pheromonic Control does you a ton of good; you do have Changeling Rogue as a high baseline for your CHA-based skills, but you don't really maintain high ranks in most of them (other than UMD and a couple levels of Bluff at the end), and so I can't tell how much you're really relying on that sort of thing. Thermoregulation is passive, so it's not something that's easy for anyone to show off here. (I would have liked to have seen Evasion to really take the biggest advantage of Thermoregulation, but we can't have everything.) I like your fluff use of Iron Stomach to eat the debris from digging your way out of prison; I'm not convinced that you can it to store poisons in your stomach, since the "any" part of it means that you'd probably just start digesting them, vials and all! (It's also not clear where you're getting your poisons, aside from Toxic Tongue and Spider Poison, which don't stick around—Craft: Alchemy doesn't make poisons, since that's what Craft: Poisonmaking is for). Still, that's cute. I do like your use of Obtain Familiar to circumvent the limitations on Feign Death, which is clever. I'm torn about how well you use Bitter Tides; your fluff painted an interesting way to use the ability to sweat acid, using it to burrow out of a jail cell, but that's very clearly a clever situation rather than a strategy that can be adapted to many different situations. Everyone suffers from how late Taste of Truth hits the field, so it's always awkward to have no Bluff advancement for 9 levels and then have a massive boost to it all at once, but at least you've got your Changeling Rogue baseline to fall back on, so it's not like you've never been able to bluff and then suddenly got good at it. You do seem to want to make the occasional full attack (even if I don't see any crazy mobility boosters to really facilitate that round after round), so Haste isn't wasted. Like I said way back on page 1 or 2 of the thread, Boiling Blood is almost strictly inferior to just a single level of Monk, so it's going to be hard to impress me with how you use it; I don't see anything here that really makes me sit up and take notice, but the ability for an assassin-type character to appear unarmed is at least giving it some lip service. As I said in Power, I'm not sure that you can use as many potions as you think you can, but you do have both Brew Potion and enough ranks in Craft: Alchemy to make use of Polyhaemia, so that's not bad. I was wondering if anyone was going to take Detach; you're the only chef who explicitly used Regeneration to qualify for anything, which is a mark in your favor, even if I would have liked to see some way of covering your vulnerability to the things that pierce it. (And yeah, Detach isn't actually that great on a Flux Adept, since they can't reattach members by holding them to the stump, but I guess it's something.)
Overall, I feel like you at least paid some token attention to using most of the SI's abilities. The fact that you used a 10-level casting class (and therefore technically didn't lose any caster levels) is nice to see, and you're not leaving me wondering why you'd bother slowing down a full caster progression for the SI's lackluster toys. Mind you, I'm still not feeling like the character needed to be a Flux Adept to do what they want to do; you wove a neat story about how Flux Adept might help someone deal with being in solitary confinement, but that's more after-the-fact justification rather than an explanation of why this character is stronger for being a Flux Adept than for being, I dunno, more Assassin and Assassin-related stuff. I'm not going to go so far as to say that being a Flux Adept didn't hurt you; your early feat progression is painful, and you definitely lose some effectiveness as a Roguish character by taking 10 levels in something with 2 + INT skills. But I feel like even if Flux Adept doesn't really make the character that much stronger, it's a lot less confusing on someone like you than on most of your fellow chefs. At this point, I'm willing to call it even; you aren't critically invested in the SI, but compared to a lot of other entries, you seem to be trying a lot harder to actually use the features you're given. 
Final Thoughts: Your fluff wasn't bad (if a bit long), though I admit that it bothered me to consider you gaining eight or ten levels while rotting in solitary confinement. In terms of what you actually do, you aren't breaking any crazy new ground here, but you're overall decently effective at what you're trying to do. I don't think you're 100% as effective as you're billing yourself to be (no Darkstalker means you're way less stealthy than you think you are, the number of spells that you can actually potionify is actually much smaller than you seem to believe, and so on), but you're not a bad character overall. I'm still not 100% convinced that Flux Adept is actually the best choice for you, but at least you're making a token effort to use most of its abilities, which is more than I can say for some of your fellow competitors.
Total Score: 12.25
Originality: 2.5 Blood Magus. Yeah. I'm not just taking off points because there were three Blood Magi. I'm taking off points because there were three Blood Magi and Blood Magus was the very first thing I thought of when I read Polyhaemia. (Plus, the fact that it's got overlapping prereqs and offers Heal as a class skill makes it a pretty natural choice for Flux Adept.) So you've got the double whammy of it being obvious and it being repeated. [–0.5] Beyond that, you're a Sorcerer, and you've got some Hexblade levels slowing your progression down, and that's about it. I have to admit that I'm not incredibly impressed, Originality-wise. I mean, I didn't see Hexblade coming, but I also don't see how it really fits the character (the character doesn't feel like a Hexblade; the character feels like a Sorcerer who happens to have Hexblade levels for some reason), so there's that.
Power: 3.0 First off, I really don't understand what good Hexblade is doing you. You seem to care about your spells, so why sacrifice three more spellcasting levels than you have to? I do understand that Blood Magus interferes with you using Paladin to get CHA-to-saves (though I kinda think a Sorcadin might have been more interesting than what we've seen here), but is Mettle really worth the loss of spellcasting mojo? Your saves are definitely above average, but I don't know if they're so great that Mettle was worth 3 levels. This really slows down the start of your build; your first few levels and your first few feats are basically entirely passive and/or defensive, and it takes a long time before you've got enough magic to make up the difference, if you ever really do. [–0.25]
I also don't see why you bothered with Blood Magus. Sure, it has Great Fortitude as an overlapping prereq with Flux Adept, but you still have to take Toughness, and worst of all, you don't have enough levels of it to get Blood Draught, the very ability that makes it an obvious pairing with Flux Adept! The fact that Blood Magus is an easy (-ish) way of getting Heal as a class skill might have been a factor if you put it before Flux Adept, but you didn't. I just don't see why it's in there. I can respect putting in something because of matching flavor, but when you don't get far enough into the second class to actually get the flavorful ability in question, I'm just left kind of scratching my head. It does advance Sorcerer casting, so it's not really making you actively weaker (aside from the opportunity cost of taking Toughness), but it's definitely not adding much to your power level. 
Your feats mostly make sense, or as much sense as can be made with the prereqs standing in your way. The first few are painful, but things mostly seem to come together starting with Extend Spell, which is probably the best metamagic feat for you to choose (aside from maybe Invisible Spell, since you lose so many caster levels and therefore don't have a ton of extra slots for metamagicking stuff up). Brew Potion isn't a terrible choice, since I definitely appreciate wanting to guarantee that you'd have access to potions to use with Polyhaemia, though it's a little unfortunate that most of your good potion-legal spells are 3rd level (and therefore expensive; you might not be worried about the 36 XP, but 375 gp a pop definitely adds up if you use it frequently). Flyby Attack is useful for nearly any caster, though it's unfortunate that you have to burn an action in combat to turn on Fly (since you never learn any all-day flight spells). Combat Casting is an interesting way of dealing with the fact that the SI doesn't have Concentration as a class skill; it's still not the strongest feat in a general sense, but it's more useful on a Flux Adept than on your average caster, I suppose. I'm not absolutely in love with Empower Spell, since I'm not convinced that your damaging spells will actually still be worthwhile by the time it comes online, but I suppose it's not completely worthless. (Heighten Spell might have been a better choice, just to make your low-level BFC spells have some more utility.) I don't see any crazy combos or anything with them, but once you get out of prereq hell, your feats mostly stand on their own merit. None of them provide a drastic increase in utility, but after the prereqs are out of the way, none of them are totally dead weight, which is surprisingly rare to see in some rounds of IC. [+0.25]
You've got a good pile of low- or mid-level Sorcerer spells, but the fact that you lose 3 levels of casting at the beginning when you know you'll lose 5 levels of casting later on is just confusing to me, since you otherwise seem to want to be a full caster. It's always useful to have someone in the party who can cast Grease, Web, and Haste (top-notch spells all around), but with all the lost progression, it seems like that's about all you can bring to the field for quite a while. You're kinda like a Mystic Theurge without the divine spells backing you up; you've still got some well-chosen spells from the strongest list in the game, but you don't necessarily get them at the right levels, and the save DCs and caster levels are lower than they really should be. Fundamentally, this is the same complaint as I made in my first Power paragraph; your Sorcerer spells are good, but Hexblade is standing in the way of you using them to the fullest. In the interest of not double-dinging you, I won't mark off again; what I'm basically saying here is that I appreciate that you made good choices with your spells known, but I don't think that you've got enough spellcasting mojo to justify a bonus for that alone. Sorcerer casting is strong enough that you might be functional in a low-op group even with the handicap you've given yourself, but that's as much as I can say in favor of your power. 
Elegance: 2.5 It's technically ambiguous whether a level 1-3 Hexblade can even make use of Craft (Alchemy), thanks to that pesky "must be a spellcaster" clause, and a Hexblade isn't really a spellcaster until level 4, so that's kinda funky. However, I think that rule is really dumb, and since a Hexblade eventually becomes a spellcaster (even if you never get enough Hexblade levels for that to happen), I'm going to call that close enough, so no penalty on that end. Still, be careful with that sort of thing.
You spent one skill point too many at level 14. You then underspent by one point at level 15, so I guess that evens out; you could just move the point in Spellcraft one level later, since that's not bumping up against any caps or any prereq thresholds. I kind of wish I didn't have to give out a penalty for such a small error (since your net total is the same), but my criteria state that your skill points have to be legal at every level, and it's a bad judge who ignores his own criteria. [–0.25]
You might have sold me on Toughness being useful at level 1 if you started as a Sorc instead of a Hexblade, but it's mostly useless for you until you enter Blood Magus, which means there's a massive level gap between investment and payoff. That's a textbook example of something I consider to be inelegant. Add in the fact that Blood Magus doesn't actually do you any good, as I discussed in Power, and we have a really graceless feat choice. Since level 1 is the only level when Toughness even comes close to being useful, I'll mark off less for taking it at level 1 than I would at level 3 or level 6 (even though that does mean you have a larger gap between investment and payoff), but I still really don't like seeing a (nearly) useless prereq feat taking up space for 17 levels. [–0.25]
Using Great Fortitude as an overlapping prereq may have been obvious, but I already dinged you for that in Originality, and it's still nice to see a prereq feat doing double duty. I really would have liked to have seen Blood Magus earlier (so as to get Heal as a class skill before taking Flux Adept), and the fact that you lose out on Blood Draught is definitely unfortunate (since that's exactly why Blood Magus makes sense to pair with Flux Adept), but at least you've got one instance of aligned prereqs. That's your only elegant way of dealing with prereqs, though; you don't have any clever ways of getting anything as a bonus feat, you don't have any other double-duty feats (or even double-duty skills), and very few of your prereq feats stay useful once you get into whatever you're qualifying for, with the possible exception of Great Fortitude making Mettle a bit more likely to come up. I definitely would have liked to have seen more on this front, but I can give you a little credit for what you did manage to do. [+0.25]
The jump between Hexblade and Sorcerer is jarring. If I were to look at your build one level at a time instead of looking at the whole thing all at once, suddenly going from Hexblade to Sorcerer doesn't seem to make sense; you do make an argument that Sorc 1 gives more and better spells than Hexblade 4 does, but it still comes off as strange to make the jump there. After a few more levels of Sorc, it becomes clear that you care more about Sorc than about Hexblade, but that just flips the confusion around; instead of wondering why you jumped ship from Hexblade to Sorcerer, we're left wondering why you started with Hexblade at all if you really wanted to be a Sorcerer all along. I think the point I'm really trying to make is that your first three levels (and the feats that attend them) don't have anything at all to do with your next four levels, Flux Adept or no Flux Adept, and that would make the character awkward to play at low levels. It almost might have made more sense if you were obviously jumping around gathering prereqs (I don't view a build full of one-level and two-level dips to be automatically "awkward" if you're obviously building up to something, especially right at the lowest levels), but you seemed to have an early build concept ("tough, resilient Hexblade") that got abandoned in favor of another one ("um, full-casting Sorcerer or something"), with the SI sprinkled on top after you qualified for it. Maybe it's the fact that Hexblade 3 isn't a super-common breakpoint (you usually see 2 levels for CHA to saves or 4 levels for the Dark Companion), so it stands out more than an obvious and common dip like Monk 2 or Paladin 2? I dunno. I just don't think your pre-SI life really hangs together cohesively. [–0.25]
Beyond that, I'm not seeing anything that stands out to me in terms of Elegance. At the end of the day, the build is relatively simple, but the different parts of it don't dovetail in any particularly clever or elegant ways—meaning that you don't have a ton of moving parts, but the parts you do have don't really seem to mesh that well with each other. It's easy to point at the different build-chunks on a high level (Hexblade provides Mettle, Sorcerer is the base casting, Flux Adept is mandated by the contest, and Blood Magus is late-game filler that's vaguely thematic), but since the different parts don't really do much to build off each other (Mettle is decent, but it has nothing to do with the rest of the build; Blood Magus doesn't really hurt you, but you don't get the ability that makes it a real thematic match, so it's just kind of sitting there), it doesn't feel especially clever or elegant, so really, we can just leave it where we are. 
Use of Secret Ingredient: 2.75 I guess we should just go down the line. Grace Through Will seems to only matter to you for Tumble, but I suppose you at least didn't ignore it entirely. I'm torn about whether Pheromonic Control is good or bad on a CHA-based caster; on the one hand, the Eagle's Splendor effect does directly make your casting (and your saves) better, but on the other hand, a CHA-based caster is ideally going to want to find a permanent enhancement bonus (that doesn't take an action in combat and that affects bonus spells per day, unlike Eagle's Splendor). I do appreciate you not assuming that you'll automatically have access to a Cloak of Charisma, but you can see where I'm coming from, I hope. Still, it's not totally useless on you. Thermoregulation is passive, so there's not much you can do to show that off. You didn't mention any crazy tricks with Iron Stomach or Bitter Tides; I do understand that those are abilities that are very situational rather than something you can showcase as a primary trick, but I don't see you doing much with them that your fellow chefs aren't also doing. I also don't see you doing much with Feign Death; it's kind of in the same boat as Iron Stomach and Bitter Tides in that it's situationally useful, but again, I'm not seeing anything special or clever that you're doing with it. You have high Bluff, so Taste of Truth is good for that; you don't have any other face skills, so it's a little weird to have a "party face" who can lie to anyone but who can't engage in honest dealings at all, but at least you do max out one of the skills that this particular SI ability boosts (although Sense Motive seems to have been ignored outright). I'm not sure what good Haste is doing you; you don't really seem to care about making attacks (though you do mention your martial weapon proficiency now and again, and you have non-dumped Strength), so I can't tell if you really care about Haste. I'm torn about whether it's good or bad that you still have Haste as a spell known; it's good because your Haste actually benefits the party and therefore really does something useful, but it's bad because it kind of obsoletes one of the SI's abilities (though I do understand that you said that you learn it as a spell known after the SI gives it to you). I'm thoroughly unimpressed by your use of Boiling Blood; as I mentioned early on in the thread, it's basically obsoleted by a single level in Monk, so it's going to take something special to make me really notice it, but I don't see you really trying to do much with it. (See my earlier comments about it being unclear if you care about hitting things or not.) You do have Brew Potion and high Craft: Alchemy, so you've got Polyhaemia covered (your argument about Tenser's Transformation and the potion of Bull's Strength involved is cute, even if it's not exactly a groundbreaking capstone ability). I'm not seeing anything that makes you especially able to take advantage of Regeneration (I would have loved to have seen some ways of getting resistance or immunity to the elements that pierce it); I guess the fact that you can heal the self-damage you do with Blood Component is something, though you would have been way better off not taking Blood Magus and instead spending the feat you spent on Toughness on Practiced Spellcaster, so far as your CL is concerned.
Overall, you don't completely ignore the SI's abilities, but neither am I seeing any way in which they really make you stronger. Since you do have Mettle, I guess Great Fortitude is a little bit less useless on you than on some of your fellow chefs, but the prereq feats still make your early levels really painful, and the fact that you've already lost some CL from Hexblade makes the half-casting nature of the SI even worse for you than for some other entries. Your build doesn't seem to be actively avoiding or fighting the SI, but neither do I believe that you'd ever have given Flux Adept a second glance if it weren't required by the contest, and nothing you do with the SI's abilities really seems critical to how the character functions. The strongest argument in favor of how Flux Adept makes you different from every other Sorcerer out there is that you take advantage of the bonuses to Bluff and Tumble, but when you consider that it interferes with your Concentration progression to the point that you need to take Combat Casting (normally not considered to be a strong feat at all, especially at ECL 15), it's kind of hard to feel great about how the SI affects your skills. In short, I don't think that Flux Adept made you a stronger character, and I don't think that it makes sense as a natural choice for the build. I just don't see you incorporating its abilities into your suite of tricks in such a way that it makes sense for you to have invested in the SI. You aren't ignoring them outright, but I still don't think the character really needed to be a Flux Adept (or even necessarily really wanted to be one), and that just doesn't make me want to give out a high score. Since you did put in a little effort for some of the abilities, I won't give out a terrible score, but I still just wasn't impressed by how well Flux Adept fit your build. [–0.25]
Final Thoughts: Your Hexblade levels are the only thing that really made you stand out from every other Sorcerer out there, but I just couldn't wrap my brain around what made 3 levels of Hexblade good enough to sacrifice 3 levels of Sorcerer spells. It's a shame that Blood Magus ended up being so common this round, and it's a double shame that you didn't get to take advantage of Blood Draught; that really makes it feel like you lost out in Originality without getting the bump from having the thematic ability that every other Blood Magus was grasping for. Overall, this character isn't terrible, and I could see them being interesting in a very low-op group (it'd be a way to bring a small amount of Sorcerer problem-solving to the table without being a full-blown T2 caster), but I never really got a unified vision of what the character wanted to accomplish and why their abilities made them cool.
Total Score: 10.75