2x6 partition walls (non-load bearing) aren’t really all that uncommon, especially when acoustical performance is desired. 15 years in construction and I’ve seen a lot of non-structural 2x6 walls.

What?

Thinset (when mixed properly) is thicker and doesn’t flow nearly as well as grout, meaning the chances of having cracks/voids between tiles is much higher. It’s also made for adhering tiles to the wall or floor, not for filling the joints between the tiles.

How are you applying the thinset before setting the tiles? It sounds like you have way too much thinset on the walls/floor if it’s completely filling the joints.

The thinset should have been cleaned out of the grout joints before it dried. You now have quite the project on your hands..

No it doesn’t. OP just needs to trim the drywall behind the face plate screw holes.

Or just use MR green board board and patch like any other drywall patch.