For d3d8, if anything installs native in a system location, that can't work and we have to prefer builtin. (I can't verify right now, but I'd guess that directx installers, which are run often especially on steam, do that.)
I see, but what about programs that ship their down d3d8 layer? Maybe it would be possible to only prefer native dlls not inside system32/syswow64, aka only those in the application directory?
Regards, Fabian Maurer