Here's what my NEW AD&D rules are. They mainly just replace clunky stuff.
12. LEVEL LOSS FROM WIGHTS, ETC.: Not permanent unless you fail a Charisma (yes, Charisma) ability score check at the each of the session for each level that was drained.
While I'm not keen on some of your changes, I have to say this one could really be on to something. (says he, making notes for next campaign's rules makeover...)
I'd still have level loss from more major undead - Vampires, Wraiths, etc. - be permanent, but having level loss from lesser undead be reversible on a save allows more undead to have such abilities; and making it a Charisma-based save is brilliant. I think I'd have it that if you fail a certain number of saves (three? five?) then it becomes permanent; you're just not going to recover on your own.
I also wouldn't tie the saves' timing to the metagame concept of the "session"; the timing would be in-game based e.g. at each sunrise, regardless of how many sessions it took (or didn't) to get there.
And one somewhat-messy question I'm already asking myself and thus I'll ask you too. Consider this scenario (xp numbers are arbitrary as I can't be bothered to look 'em up right now):
Jocantha meets a Wight and loses a level, going from 5th (at 24,367 xp) to halfway through 4th (at 16,001 xp).
Later that day she gets into all sorts of mighty endeavours and in so doing gains 2300 xp, not enough to bump her back to 5th (at 20K).
Next morning she makes her save and the level loss is undone.
What happens now? There's four possibilities, and an argument can be made for each:
She wakes up at 20,001 xp (i.e. back at 5th but right at the start of the level, much like a Restoration)
She wakes up at 22,301 xp (i.e. back at the start of 5th plus what she earned after the loss)
She wakes up at 24,367 xp (i.e. back where she started, overwriting her later gains)
She wakes up at 26,667 xp (i.e. full recovery plus keeping what she earned after the loss)
And a corollary question: what happens if a Restoration spell is cast on Jocantha in the meantime?