I think you've slightly misread me here. It's not that we can't lock down what his floor above 21 is, it's just that there are multiple options to pick from. I am open to any of the numbers suggested by people here, which include 26, 27, 28 and 32, with different reasoning. I think the debate should be which of those numbers represents the right floor, and we should then stick a + after it, to indicate it could be higher. To me Level 27 seems the appropriate way to calculate it, in line with how other OOTS characters are getting calculated, but I'm open to any of those other numbers. What I don't really think is fair is to peg him as only 21+, which some people admit they are pushing because it "seems weird" that he'd be so high. What I did say was we can't pinpoint it using his Epic Spells, because the DC is too uncertain to use as a level calculator, but the Epic spells form supporting evidence to back up the other evidence I cited, all of which suggests Xykon being around level 27 or higher isn't inconsistent with the story narrative at all. Quite the reverse.
Again; if V used Quickened Cone of Cold next comic, everyone here would nod and agree he had hit level 17. Xykon's Maximized Energy Drain should get the same treatment. You can construct elaborate scenarios for why both wouldn't apply, but like V (or the examples I gave of Roy or Miko) we shouldn't do that, we should apply Occam's Razor and go with the most likely one until there is evidence to the contrary. The item that grants immunity to meteor swarm and the DC of his Epic Spells also support the Occam's razor approach to him being level 27.
As for the Ring; can this non-epic ring be crafted to provide full immunity to both fire and bludgeoning damage? I'm genuinely not sure, but I'm guessing the answer is no.