1. Wouldn't there be any technical problems with creating new pages for new users, if permissions are set so that only admins and mods can create new pages (as mentioned above)?
2. If 'user profiles' is disabled in the site manager and someone leaves the site, their profile page would still need to be updated with the _former tag… in case user profiles is enabled again at some point in the future

1: It's not a problem. Wikidot can do it.
2: No. When profiles are disabled there must be no magic happening on any category. The problem you described may be fixed by (automatically) updating the tags of profile pages when they are enabled again. I'll update the design to include this issue.