I have been following this thread. You won't be able to use the mailman shell due to this being a shared mailman 3 server. Just let me know what the email address is of the user that has an outstanding pending request and I should be able to clear that out via Mark's instructions. You can do that via our support ticket system.
I also wasn't aware of a setting for expiring pending requests. I will wait for Mark's response to look into that further. I can tell you from experiencing that making your subscription policy open will not allow those with pending requests to be added. I just tried that myself.