I,ve had a problem like this before on a wet room that I had done,first thing poured water over the floor no leak,then poured water on to the walls no leak, it turned out to be the shower control,might be worth checking that.
I started reading this thread last night and was thinking along similar lines. Its quite a wide water mark down the wall. You say it leaks when water splashes the wall. Have you checked its not a leak on the outlet of the valve? Which ill presume is built in?