This is indeed an oversight, EGL_CFLAGS should be added to UNIX_CFLAGS in the Wayland driver.
Yes.
I guess the assumption here is that in case of such setups, the dynamic linking runtime environment would also include the proper paths, but of course it's not guaranteed.
You can probably ignore this part of my comment. I had in mind that SONAME might include the full path of the libraries, but it does not, so wherever pkg-config ends up locating the libraries it will end up with the same SONAME (which still needs to later be loadable without a path, but that's probably expected).