1. What does this do? Something called mailpasswds is kind of scary...
/usr/local/cpanel/3rdparty/bin/python -S /usr/local/cpanel/3rdparty/mailman/cron/mailpasswds
2. Why is it failing with this error?
Site list is missing: mailman
That's a Mailman script, as opposed to any custom cPanel script/integration. Here is it's description:
Send password reminders for all lists to all users.
This program scans all mailing lists and collects users and their passwords,
grouped by the list's host_name if mm_cfg.VIRTUAL_HOST_OVERVIEW is true. Then
one email message is sent to each unique user (per-virtual host) containing
the list passwords and options url for the user. The password reminder comes
from the mm_cfg.MAILMAN_SITE_LIST, which must exist.