One thing that annoyed me about 2e Thieves in particular is that in order to put points into a Thieving Ability, you had to use it. This presented a real issue for me- if you invest a lot of points in some skills so you have better than a coin flip to use them, that leaves you with skills that are hard to improve later...hm, in retrospect, I'm reminded of playing Morrowind or Oblivion.
Anyone who wants to make playing a Thief a less unpleasant experience has my vote. So I like what I'm seeing here, though I have a few caveats (all based on personal opinion, so heed or ignore what I have to say freely):
*I think there's no reason to lock out any class from learning how to hide, move silently, climb walls, or detect noise. Academic types really should be able to learn how to read languages. Turning the classic skills into NWP's , then giving Thieves bonuses to them and free proficiency feels like a better approach. After all, look how many free NWP's the Bard gets!
*Backstab is entirely too persnickety. Depending on the DM and how closely they follow the various rules for the ability, it can be something you can use often or so rarely that it's not even worth marking the ability down on your sheet- my last 2e Thief fell into this category- they were a Dwarf with the Locksmith kit. I wasn't sneaking around, I was there to find traps and open locks. I'll just hang back and plink things with my shortbow, thanks, so I didn't even write down that I had Backstab (to the horror of my fellow players, lol) as I had no use for it!
*The ability to use consumable magic items is similarly DM-dependent. If your party often finds such things, then you can expect your Thief to have a stockpile of them- but in this case, the ability should come online sooner. If your party rarely finds such things, or they quickly get snapped up by the casters (I mean, what Bard or Wizard is going to share a scroll he could scribe into his spellbook? Plus Bards are better at scroll use than Thieves, to boot!), what seems like a major ability is basically pointless. I played a Priest with the Magic Domain in 3e, thinking I was going to be able to use Wizard scrolls all the time, and ended up using the ability twice by the time I was level 9.
*Consider adding Evasion to the class. It existed in Oriental Adventures, so it's not really out of place in AD&D. It really helps sell the nimble guy leaping out of the way of certain attacks. Giving them a better Breath Weapon save certainly helps (sure, Dexterity's Defensive Adjustment helps, but not every Thief is going to have super high Dexterity!).
*Don't forget that Sneak Attack encourages dual-wielding characters over ranged characters, putting the Thief in more danger thanks to their terribad AC. It's ok if ranged Sneak Attacks have limitations, but at least give the Thief the ability to fire into a melee without accidentally Sneak Attacking an ally! Further, the fact that Sneak Attack turns off against a wide array of foes, including common ones like Undead, is particularly odious in play- though I admit, that's hardly a unique thing in AD&D- Turn Undead, Favored Enemy, etc. etc..