> The wchar_t strings themselves are built with mbstowcs(), 
> the file system encoding is not used.

Oops sorry, you are right, and it's worse :-) sys.argv is decoded using the locale encoding, but subprocess & cie use the file system encoding for the reverse operation. => it doesn't work if both encodings are different (#4388, #8775).

The pseudo-code to create sys.argv on Unix is:

 # argv is a bytes list
 encoding = locale.getpreferredencoding()
 sys.argv = [arg.decode(encoding, 'surrogateescape') for arg in argv]