Saving her now, by taking a young idealistic boy on an important high-stress mission, doesn't seem like a good plan.

Unless the primary purpose is to change Harry. Duping Harry into rescuing Bellatrix Black creates some pretty hefty blackmail- most importantly by Harry against himself. Harry can name the fallacies involved, but that's no guarantee he can overcome them.

Remember, pretty much every action Quirrel has taken so far has been pedagogical. It seems far more likely that he's grooming Harry than that he's rebuilding his power base.