If I wear Drizzts +4 mithril chain mail, can I use thieving abilities and cast spells?
Yes. I went to the trouble of killing him just to test it. Looks spiffy, too. Q: Bjornin’s quest is a pain. I can’t find the half-ogres, and can’t be sure I killed them all. A: Same map as Drizzt, above. If you don’t want to trigger the Drizzt encounter, stick to the top half of the map. I’ve tried this and, by diligently sweeping the top half, fulfilled Bjornin’s quest. You don’t get direct confirmation on this until you get back to the Juggler and see Bjornin, though Detect Evil would answer the question.