Thanks for having me on this very informative forum.
I have a set up very similar to the pictures shown here,
My problem is that I'm getting water leaking to the bathroom below in the basement while the upstairs shower is running.
I suspect that the valve stems have been due for replacement for sometime.
Could worn valve stems allow leaking while the water is running?
(I'm hoping that is just the stems, and that I don't have to replace the whole brass mixer assembly.)
I'm sure I've revealed my complete newbie status of plumbing knowledge... please school away.
Most likely it is repairable without tearing up the wall. For your detective work, remove the handles and the chrome trim. Also remove the trim flange around the shower head. Leave the spout as is for now.
Now, reinstall handles and turn on water, use flashlight to inspect inside the walls around stems, shower arm. If nothing is discovered here, the leak could involve the tub spout, and then you just have to remove it and start over.
Now, you didn't mention exactly where you see a leak. If it is out in the middle of the ceiling below, it can also be associated with the tub drain. That will need more detective work, and ultimately may need opening the ceiling, but try the other things first