> he himself puts double quotes or calls CommandLineToArgvW

CommandLineToArgvW has nothing to do with creating a command line for use with CreateProcess. It's used by a process to parse its own command line using the common VC++ rules.

> subprocess.list2cmdline() is always called

No, it's only called for an args sequence, not a command-line string. If you have your own function to convert a sequence to a command-line, then call it explicitly, e.g. subprocess.run(mylist2cmdline(args)).