I like this mod and the difficulty, but somehow the resistances and imunities every tough enemy has, forces me to metagame and look for the .cre files which attack/spells aren't pointless against them. Because nearly everything (and nearly every strategy) is pointless. ^^ Are this heavily immunities really necessary? I feel like my thief and 97% of my speelbooks are absolutely worthless against the IA enemys.

(Possible) Bug: Borinall backstabbed my skeleton warriors. And im pretty sure theyre supposed to be immune to backstabs.

No, it's not a bug. Skeleton Warriors are not immune to backstab.

Is this AD&D rules? Because in 3.0 and 3.5 undead creatures are immune to critical hits and backstabs. I'm not sure about AD&D.

In BG2, Skeleton Warriors are not immune to backstab. In 2nd edition AD&D (which is the nearest rule set for BG2) they are not immune either. The only difference between BG2 and 2nd edition (regarding skeleton warriors) is that in 2nd edition, Skeleton Warriors are immune to turn undead, while they do not have this immunity in Bg2. Improved Anvil's "Skeleton Lords", however, are immune to turn undead.