The "dwarf with lots of hit points and a ring of regeneration" method can work just fine as a thief as well, by the way, using the Knock spell to get past doors and opening chests that you cannot bash open.
See if Viconia can cast Find Traps. Send Haer'Dalis over them with Stoneskin and Mirror Image to disarm them. Haer'Dalis can pick pockets and cast Knock to unlock chests and doors. In place of Thief traps, use Skull Traps, Glyphs of Warding, Delayed Blast Fireball, etc. In place of the thieving ability Detect Illusion, use True Sight, True Seeing, and other divination spells. Instead of backstabbing opponents, beat the crap out of them with your Fighter and Korgan, or have Edwin cast Time Stop and do anything you want to them. What else do you need a Thief for?
Yoshimo/Imoen also fits the bill in your party. Sure, Imoen is Good, but she's your childhood friend and won't leave your side no matter what your Reputation is.
"A life is not important, except in the impact it has on other lives."
-- Jackie Robinson