My client has supplied Mandarin Stone 10mm thich polished & honed slate for the shower encl. and floor.

Back wall of the shower is stud, main end wall is solid with plaster, other end wall yet to be built.

Few questions I need help with?

1) What to overboard the back wall with?
2) Is it ok to tile onto the plaster of the main end wall? Dont really want to overboard as the edge will be visible
3) What to build the other end wall of, ply or hardibacker or wedi?
4) Do i need to seal these polished ones before laying?

Mandarin stone are saying the shower needs a waterproof backing, assume you could use ditra?

Ok to do floor onto 18mm WBP? Joists are good.

Help would be appreciated, first slate job!!!