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I'm afraid there are no answers to your questions short of 
doing an exhaustive study of each implementation of 
interest, and even then it hits a brick wall because Win32 
CreateFile() doesn't document its error conditions and its 
source code is secret.

I'm attaching a patch that should reliably distinguish 
between mode and filename errors across Windows flavors 
under MSVC, and that maps EINVALID to ENOENT on Windows 
(which may not be the best that *could* be done on Win2K, 
but Windows flavors are inconsistent here and Python has no 
idea which flavor of Win32 it's running under).  If that's 
good enough for you, I'll check it in,; else I'm inclined 
to close this as 3rdParty and Won'tFix.