Hi, I'm going to browse through and see what I spot. Can't guarantee I see the full picture, but I hope that we can get talking about things and then go deeper as needed. I'm writing this as I go, so things might make more sense later on... we'll see how I edit this.

1) hitdice: 2d4 - an odd choice, but an interesting one. I assume this doesn't just mean a different way to calculate your HP at level up, but also more HD in the short rest category (and probably features I'll be reading about shortly)
I've done Hit Die like this with a couple of classes I've created or reworked (namely the ranger). Indeed, it does provide more versatility during a short rest (e.g. one can use a "half Hit Die") and also allows a larger pool of potential Hit Die for powering class features if needed. I do like how it ends up working for the most part. I did consider using Hit Dice to empower features with this class initially but backed off in the first draft with the exception of being able to front load your skinchange shape with pre-heals.

2) Multiclassing. While you mention what stats you need to multiclass, you don't mention what skills/etc you gain when you take your first multiclassed level of skinchanger.
Corrected. Thanks.

3) "your mantle can't be forcibly taken from you" <== does this mean it's literally attached to you? How about damage? (This is a risk with building a class with an item as basis. Maybe go the Warlock route where you can recreate the item? Or even just assume the item is an idea and not a physical one for most of it?)
Ugh. Problematic. Corrected. I did have the blurb about recreating at the end of the feature but it makes little sense if it cannot be removed/taken/destroyed.

4) Is the speak with animals part also limited to animals associated with your mantle?
No, not intended to be. Allows the ability to speak with all beasts as per the spell parameters.

5) Blood scent: Interesting (feels similar to certain sharks I think?) However, in a more "lived in" universe... how often won't there be a random hurt bird, insect or other critter in that radius? The usefulness of this ability will be very much based on how realistically the DM will handle it. After finding 7 dead rats, your player will stop using it. But if you find that hurt enemy 1 mile away... what kind of nature is naturing around there?
Seems so obvious now that you pointed out the flaw. Reworking this feature.

6) Primal rage seems... powerful at low levels. Both the extra attack and the regeneration. Granted, the regeneration might be weaker than resistance to physical damage... in certain situations, but the extra attack will often overpower the rage damage bonus (comparing to barbarian). So yeah... not sure about the balance on this one.
I did run a simulation against a barbarian at levels 1, 5, 11 and 17. Both classes wielding a great axe and starting with a 16 Str (ending with an 18 Str). The Skinchanger does have an edge at 1st level but after that it's mostly equal with the barbarian actually being slightly more powerful in terms of base damage at the highest level. The barbarian's second melee attack with rage bonus really evens things out. Averages were as follows: B1: 11.6, B5: 25.0, B11: 27.0, B17: 29.1; S1: 16.0, S5: 21.6, S11: 24.9, S17: 28.5. It's always hard to model these things as so many other variables come into play that aren't represented.

I did go through and rework the Primal Rage feature. The extra attack now has to be one of the mantle's rage attacks which also have had their damage reduced (this is what the damage output model were based on).

7) keen senses: the first part seems okay, basically a non scaling proficiency bonus to perception (due to being a different bonus, it might go insane if you do take perceptive, expertise and other fun things). The second part though. That's quite the powerful ability. (Though technically this doesn't affect hiding/stealth, but probably intended to affect it none-the-less). This pretty much says nope to rogues and most other forms of ambush. Which seems over the top to me.
Yeah, too powerful. Reworked.

8) Lesser skin change. The 10 minute to transform and then 1 hour duration seems harsh... maybe make it 1 minute (enough to make it non combat viable/roleplay interesting, but not quite so long). Beyond that, the temporary hitpoints/transforming back isn't described properly. I assume it works like a druid "new shape HP, and if you drop below zero, you take the rest of the damage in your normal form"? I'll probably be back to this feature once I get to the mantle part of the document.
I can buy that the transform time is too long (will reduce 1 minute) but the duration is as intended. I've tried to better describe the feature in the forthcoming rewrite. It's somewhat an homage to the 2E druid and how shapechanging would heal them.

9) ASI: the text says normal (4,8,12,16,19) while the table has fighter style (4,6,8,12,14,16,19) - which is it and why? (and if you go normal, maybe move one of the level 7 abilities to level 6, etc)
It was intended to mirror Fighter/Rogue ASIs but I missed this on the feature explanation. One of the hazards of copying between multiple templates I suppose. Originally this class was much lighter on features and my perception of its power curve was that it was light -- thus bumped it a bit with the more aggressive ASI progression. The way it ended up, this may not be needed though it does make sense (to me) from a thematic perspective.

10) The 10 to 17 range of levels are interesting, but a bit heavily stacked. Not quite sure yet and not going to look to deep into it at this point in time.
Really 11th and 17th are just iterations of the 5th level feature (Skinchange). Frenzied Summons was really the only bolted on addition. Depending on what is done with the ASIs, I may have room to smooth this out as needed.

11) At level 18 you are basically immortal, aren't you? To everything except time... though technically even that isn't true from what's written.
Well, I'd argue that almost any 18th level PC is immortal but I get your point ; ) Revising this feature some to have some downside (and storytelling potential).

12) the powerful build is good, the added two handed weapons... might be a bit much (and the heavy property won't even count for most characters)
Heavy is redundant probably, but it makes sense if nothing else (?). I think with the reduction of the damage die by one step it's fine. The intent behind this was to allow the use of the special rage attack if wielding a 2H weapon.

13) wild summons... I'd do this more like a normal summon spell. So lowest amount of creatures and lowest CR at level 3. Either more creatures or higher CR at level 5, etc. As 6+prof boars at level 3... is going to seriously add some muscle to a party.
Admittedly this one got kind of messy. I do need a clearer way to handle the progression here.

Anyhow, long story short, my conclusions:
  • I like the idea and feel alot.
  • There's a bit too much going on for this class at many levels (lots of "good ideas" + "extra bonus for me" feeling)
  • And finally, the combination of rage+shapeshift+summon might just be a bit too much at the same time.
Thank you for the comments. Very helpful! Some of the things I can't believe I missed given how many times I proofread this damn thing. I don't disagree that it's possible the overall combo/synergy between rage+skinchange+summons might be a bit much but was hoping that it balanced out a little in that they all draw from the same pool and recover on a long rest. I did tweak (slight nerf) rage some which will help a teeny bit.