Disclaimer: this is purposefully obtuse.
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.
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.
You seem to not be understanding so let me break it down.
Playable - you can play as this
You have a list of playable character options in the character creation section of the phb, and additional ones provided by sourcebooks.
If it isn’t a playable character option, it isn’t playable. RAW. It isn’t an option. It isn’t playable.
RAW. Let me get that through your head.
“Rules as written”
Not RAI.
Not “rules as intended”
RAW.
Written.