But, (as you know) this still won't filter out other user accounts I don't want listed in the statistics member count or the member list because "UserGroups" can't be assigned automatically when a new Wordpress account is created.

You also can set the default usergroup in the same admin page. The default usergroup can be hidden. So all new users become hidden until admin change the usergroup to a visible usergroup.

These are all wpForo can provide for your issue. There is no other options and settings.

Possible bug? I just turned off "Display on Members List" for my admin wpForo Usergroup, hit save, and the checkmark keeps coming back. Tried multiple times. (Using Chrome)

Yes, we know about this bug. This is only Admin usergroup issue, others work fine. This is already fixed in next release.