@Braxien I don't think what you are seeing is uncommon for Britechester lots that have been converted from Uni Housing to Residential. The game seems reluctant to treat these residential lots the same way it does for lots in other worlds. I'm not 100% sure but it seems like most of my uninvited guests are part of some event occurring on campus, like student organization events, sports events, etc. The bots from the Bot Savant events will sometimes stay at my Sim's house for days at a time. That's the Foxbury campus though so it would be different than what you are seeing at UBrite.
As far as invited guests not seeming to want to leave, yeah I have seen that too. My young adult Sim's mom shows up for visits several times a week and sometimes stays until well after midnight until I banish her with MCCC mod commands ;-D The mom is an NPC since my Sim moved out of the family home to start Uni. I've never just let it play out to see how long she'd stay otherwise but I might try it the next time I play that save.