Can I use Power Grab adhesive when installing tile on shower ceiling. There will be Durock backer board installed. If so, can I use dabs of the adhesive, or does it need to be spread over the entire surface of the tile?

Why can't you use mastic? I'm not familiar with this power grab

Since you should be using "real" thin set mortar (powder) for your shower walls and floor, why not use it for the ceiling too? It's also the best product to use. I bet you're afraid the tiles will fall. No they won't. Just mix it right, nice and thick but creamy. Might be easier to spread on the backs of the tiles. Apply, push & slide back & forth to create a suction, and you should be all set.

BTW, you do not absolutely need cement backer on the ceiling, it's great if you want to, but not required. How are the walls and shower floor going to be done? What type of waterproofing and moisture barrier method will you be using?

Jaz