Replacing drywall on walls - keeping ceiling drywall

My wife and I recently decided to redo our master bath. We tore everything out completely with the exception of our ceiling drywall. Which leads me to a few questions:

1) How should I go about taping the seams between the fresh drywall on the walls and the ceiling (the ceiling has multiple layers of paint on it). I've torn the previous layer of tape from the ceiling, but I'm wondering if I should "rough up" the ceiling a bit more in order to give the new mud something to stick to.

2) How should I treat the joints between drywall and cement board on the walls? fiberglass tape and mud? or fiberglass tape and thinset?

3) How should I treat the joints seems between the cement board and the ceiling? Silicone? I'll be tiling all the way to the ceiling in the shower area.