1.) Do you apply the thinset to the tile first then put on the ceiling?
2.) What type of thinset do you recommend for ceiling application?
3.) What type of trowel do you recommend (U or V shaped)? How big?
4.) How much thinset do you apply to the tile or backerboard?
5.) How do you grout the ceiling and get the grout to stay up and not make a big mess all over the place?
6.) What type of grout do you recommend?
7.) I am going to use the same 13x13 tile on the ceiling as the walls. I plan to use 1/8" spacers on the walls. Is there a problem with using 1/8" spacers for the ceiling also? That way all the grout joints line up.
8.) The place where the ceiling tile meets up with the wall tiles, do I grout that joint? Or do I just use caulk? Also I purchased a wet saw that does 45 degree cuts should I cut the corners and join them together?

Actually have never seen the tile applied to the ceiling in the shower, that is usually painted. Why are you doing it that way ?

Levitation maybe...

1-4) You generally apply tiles to the ceiling the same way you would do it on the wall. With tiles that size, you might need to back butter to insure good coverage. The thinset will keep the tiles up. To reassure yourself, push one up on the ceiling when you are doing the walls and try to pull it off. On the ceiling they won't move after you apply them, unlike the wall where they tend to slide down.

5-6) Thinset and grout should be mixed from a powder and not premixed for showers. The normal texture grout should be thick enough to work on a ceiling, although there will be a mess with some falling.

7) Spacing is a matter of taste and the regularity of the tiles.

8) All changes of plane should be caulked, not grouted.