When the valve is in the OFF position, no water leaks. So the problem
is not in the soldered joints. These are 5 years old and have not been
a problem. However, when water flows up to the shower head, it pours
down inside the wall.

Kohler has been very helpful and has sent me all the internal parts.
But it behaves the same after installing them.

The valve appears to be a red herring. There must be a leak in the pipe
from the valve to the shower head. Can anyone suggest a way to inspect
this and/or fix it without removing the tiles? HNY.