Yes I performed the test as well, can confirm your result. Despite using the JP file in both location, I still got the JP layout.
Still, JP players are getting the western one.

This suggests me that the game, when Xinput is activated, uses some sort of fixed/hardcoded layout. This fixed layout is swapped for JP and EU/US region, and is hardcoded somewhere else, not in the xinputdll.dll file.
Now if only we could find WHERE, we could take that file, swap it with our own, and maybe it would get the western layout...