> I appear to be somehow getting child processes where __main__.__file__ is
> set, but __main__.__spec__ is not.

That seems to be true for the __main__ module even when multiprocessing is not involved.  Running a file /tmp/foo.py containing

    import sys
    print(sys.modules['__main__'].__spec__, sys.modules['__main__'].__file__)

I get output

    None /tmp/foo.py

I am confused by why you would ever want to load by module name rather than file name.  What problem would that fix?  If the idea is just to support importing a main module without a .py extension, isn't __file__ good enough?