Other effects in the game which explicitly state they kill you:
Shadows, succubi, massive damage, death saving throws, beholder death ray (notably not even their disintegration ray kills you), power word kill, vampires, mind flayers, night hags, drow inquisitors.
Clearly, if they intended for disintegration to kill you, they'd have said so. Since specific overrides general, and there is no general rule that disintegrated creatures are dead, I rest my case. QED.
I mean, this is a game where we are being intentionally obtuse. I don't care about the game in any capacity. I care about what is written in the rulebook.
Feels like you may have lost the plot of your own joke post.
The target’s game statistics, including mental ability scores, are replaced by the statistics of the new form. It retains its alignment and personality.
OP could've just repeated "spells do what they say they do." It doesn't say you lose control of your character in the new form, all it says about the new form and how that affects the character is...well, that line(plus a few other things about the gear they were wearing and whatnot).
If it makes a character into a non playable character option, what rule is suddenly making that character playable? Nothing explicitly states that you can play as a character that is not a playable character option. You have a set list of RAW character options.
But nothing explicitly states that you stop playing as the character you were playing as if they were transformed, whether into a playable character option or not. There is no rule saying that that character is playable, but there is no rule saying you can't play as them. Again, spells do what they say they do.
No, you're not understanding.
There is nothing saying, RAW, you can't play as a gnoll. The RAW is that you can't choose a gnoll as your race - but in the case of being polymorphed into one, you didn't choose that, you were changed into one. And the RAW for that is to assume that everything works the same as before, barring what is explicitly said in the spell for that transformation. And once again, nothing in the spell says you stop playing as a character who is transformed, regardless of if the transformation is "playable" or not. There is not even a "playable" tag for anything, anywhere, in RAW.