I didn't run the script.  I have now, but I'm not clear from its output what each test is actually doing, and don't really have the time to figure it out from the code right now.

I think it is probably more efficient to just ask you what your suggestion is for making things more portable?

As for the docs, the docs link to the os.exec python docs, which explain the PATH semantics.  Linking to the Microsoft documentation would equivalently explain the Windows semantics.  An explicit footnote discussing the differences in PATH behavior in the subprocess context would probably be helpful.