Unless this is a steam shower and it doesn't appear to be one, you don't need the Kerdi on the ceiling, as for 16x16" slate, I have never installed that on a ceiling yet, I would use cementboard screwed to double the 3 of braces as normally would be up there, install the slate with a good modified thinset and put 2xs across under them to brace it overnight, just in case, maybe others can add to this.
Heck, we use Cedar outdoors for decks all the time, don't see why it can't be used on that ceiling, again, let's see what others say.