[Faceman]

I've decided to start (beside my evil Assassin) a new game with a good-aligned character (not sure about class yet) once I've downloaded BGT.
Now I want to get Imoen back as fast as possible both for powergaming purposes (I want a decent mage FAST) and for role-playing (I just miss my sis so much).
I have finished the game often but I've always done the essential Chapter 2/3 quest before I got Imoen back (resulting in a lack of experience with her). This time I'll try to get to her ASAP only doing one of the quests (e.g. the keep) if any (maybe I raise the money other ways). But I'm afraid and curious.

Which quests can still be done after chapter 5 and which will be unavailable or screwed up? And what will I miss (monsters, equipment) during chapter 4/5/6 if I do them low-level?