You can mix up some thinset a little runnier and skim over the top with the flat side of your trowel to lock all the loose sand. It’s supposed to be sandy, that’s completely normal. As cement (in the thinset) cures it grows little crystal ‘fingers’ which lock into whatever substrate it’s installed upon. This locks the thinset into the substrate as well as the back of the tile. It will adhere just fine. Not only does the cement in the thinset do this, the cement in the deck mud continues to do this as well (mostly in the first 28 days) so they essentially lock together with a crystal matrix.

Didn’t know tile had any connection to Star Wars, did ‘ya?

You’ll need to create a flat plane on your shower floor if you want to use slab granite there – which will require a linear drain at one end of it. Keep in mind as well, one large slab of granite on your shower floor is going to be extremely slippery. I would not recommend that at all.

The shower wall can be prepared in any manner you choose just as if you were installing tile. Any of the membrane methods will work. You don’t need to do anything special with the ceiling at all.