Keep in mind, for this i did modify my character w/ shadowkeeper in BG and exported him to BG2.

now, it seems that I'm having trouble finding some secret doors and starting some conversations, and i am not able to backstab at all. The only thing that i can see that would interfere is a helmet that thieves technically cannot used, however that shouldn't be a problem because i can still lock pick, find traps, and hide in shadows, right?

What armour is the would be baclstabber wearing? And what kind of enemy is he trying to backstab?

Studded leather armor, and i've tried backstabbing ogres, duerger (in Irenicus' dungeon,), and other minor characters, i've done it while "hidden" in shadows

Have you tried pausing while hiding behind your victim and queue the attack for when you unpause?

Did you edit any of your thieves abilities ?

Also have you tried removing the helmet to make sure it's not causing the problem ?

Yep, looks like a bug, i created a completely new character and tried to backstab and and it didn't work. Thanks for your help