This is a typical case of a tile setter trying to be "too good". They sometimes don't understand that a much larger hole needs to be left exposed for maintenance. That is why shower valves have large trim flanges....to cover the necessary hole. You will probably need a hole about 1½" diam. minimum in order to remove the stem with a stem wrench. If you could post a picture of the set-up on photobucket or similar, we could probably lead you to an on-line parts diagram.