And giving a super-powered character reasonable limits rather than catering to minmax fantasies isn't necessarily wrong.
Reasonability is exactly the problem. The writers never specifically established what limitations the priest's power might have. They didn't provide enough setup to justify the payoff. I resent the phrase "minmax fantasies." Unreasonable extremes are exactly what I don't approve of. I'm just applying logic to what I'm shown. If you think about it, shapeshifting logically implies extreme control over bodily structure. Closing up a hole in a wounded body part is a much, much simpler and less drastic transformation than changing an arm into a wing or skin into clothing or an entire body into water or dust or sand or mist. Therefore, it simply stands to reason that if you have the power to shapeshift
at all, the logical consequence of that is that healing yourself should be easy. Especially if we're talking about full-on self-disintegration. For most people, being entirely disintegrated on a particle level would be the most irreversibly fatal thing imaginable. So if you can survive that, then being shot should logically be trivial in comparison. Shapeshifters with self-disintegration ability should be all but indestructible, and that's usually how they're portrayed in fiction. The T-1000 could only be killed by extreme heat. A Dominion Founder could only be killed by a high-powered energy weapon or by disease. Sandman can even survive being melted or turned to glass as long as the single speck of sand housing his consciousness survives. Hydro-Man can be completely evaporated and yet still reconstitute later.
So if a writer wants to posit that a self-disintegrator character cannot easily heal, it is incumbent on them to
explain that inconsistency -- as, for instance, with the idea you and I
both proposed that perhaps he instinctively/reflexively reverted to whatever his physical state was before transformation. (Evidently you didn't read my post from yesterday and didn't realize you were proposing things to me that I already proposed myself.) But no such explanation was offered. It was merely tossed in arbitrarily, like everything else about how the powers work in this lazily conceived franchise.
So there's no "minmax fantasy" here, merely logical analysis. Good grief, controlled self-disintegration is probably the most "minmax" ability there is to begin with, short of turning yourself completely into energy like Livewire or the Living Laser. It's one of my least favorite superpowers because it's so utterly nonsensical and effectively magical. So if you're going to give a character that kind of power, then they should be portrayed as astonishingly powerful and nigh-indestructible, like the T-1000 or Sandman. Giving the priest such a high-grade ability and portraying him as vulnerable at the same time was an inconsistency they should have justified, but did not.