The rules have Resolve Attack as a single step. Damage is part of that, yes. Nothing in there says anything about having to go though the full damage before any other part of the attack is resolved.
The damage done by an attack is a single amount. There is no suggestion that it is applied in single hp increments.
Your arguments here remind me of the time when you, under your 'Orethalion' username, got me to stop contributing to the WotC threads. Remember when your argument was that the
effects of a spell appear
before the spellcasting (VSM components) has even been completed, just to try and justify your idea that you can cast
some of an 'instantaneous' spell, see what happens, and then decide who to target with the rest of the same instantaneous spell? When you challenged the rest of us to prove you wrong, I realized that you'd gone through the looking glass and I wasn't going to follow.
Here, you're choosing to change the way damage is resolved, just so that you can willfully misinterpret the rules so that you can say that they say something that you know that they don't: in that case 'how instantaneous spells work', in this case 'how damage is applied'.
That's not true. The English language doesn't require "this damage" to be full damage.
In this case it does, because 'this damage', whether it is from a spell or a sword stroke, is a single amount. A sword stroke doing 12 damage doesn't hit you 12 times, and a
disintegrate doing 80 damage isn't 80 spells with a dust check after each one.
If 'this damage' is 80 points, then 23 points is not 'this damage'. The spell does 80 damage, 80 damage causes the beast form to lose all of its 23 hp and the reverted druid to lose 57. If you don't apply that 57, then you haven't applied 'this damage' because that 57 is the damage done by that spell just as surely as the other 23.
You think that you are allowed to just do 23 and ignore the other 57, and require us to prove you wrong? Okay, I'll choose to take, say, 1 point and ignore the rest. Prove me wrong! Show me, in the rules, exactly where it says I'm not allowed to do that!
The burden of proof lies with those who are claiming things the rules don't say. They don't say that you can choose not to apply all the damage.
Bad analogy. A good analogy would be if the loan shark was going to break your legs if at any point you have no money.
It might not be the best analogy, but loan sharks care about getting their money, not about the adventures you had in order to get their money.