Divisions can be complicated. For the full picture, see:
They effectively create silos; you can collaborate among Division members within a Division, but not across Divisions with premium features.
You will pay for members in the two Divisions you mentioned. All others will be in the free Asana Basic plan.
Yes, you would have to move the projects. But you are not limited to one team. In each Division’s admin console, you can designate which teams are in the Division, so you may have more than one.
Yes, I believe so.
Same as 1.; or did you mean something different?
Again, this is complicated, and the above is the best of my knowledge, but I recommend you confirm with Asana Sales or Support before you change. And perhaps others will weigh in here; cc @Julien_RENAUD, Division Czar!