> I fear the potential 3rd-party breakage alone should bump
> this to its own issue.

The change to the DWORD converter in _winapi should only be in 3.11+. If this causes problems for other projects, they're probably depending on undefined behavior in the standard library. No third-party package or application should use _winapi directly.

> 1) modify Windows Popen._wait() to raise on out of bounds 
> values [< 0 or >= INFINITE]

I think raising ValueError would be best at this level. For example:

            if timeout is None:
                timeout_millis = _winapi.INFINITE
            else:
                timeout_millis = int(timeout * 1000)
                if timeout_millis >= _winapi.INFINITE:
                    raise ValueError("'timeout' exceeds the platform limit")
                if timeout_millis < 0:
                    raise ValueError("'timeout' must be non-negative")