Well first things first. Good luck.
1. Sounds okay to me. I don't like dual classed characters and I can't stand Jaheira anyway but that's just me. If you've got to fourth/fifth level with that group you'll probably be okay for the rest of the game.
2. YES!!!!!!!! I wouldn't dare go to Durlag's tower without a sixth/seventh level party. BUT, you can always go in and then come back out again if it get's too tough.
Where exactly in the game are you? Coz I'd advise being almost at the end before starting anything in the expansion.
3. Ummm. Good question. Can't think off hand. The crossbow of speed and shadow armour might be a good idea. Other than that, just the best armour your characters can use and some nice destructive spells for your mage.
Have fun!
P.S- guessing by the name that you're a Robert Jordan fan. Very good taste
