Twallan got back to me and while there is still one point open (the sims getting stuck later when they got to bed), he was able to pinpoint the reason why the girl turns invisible.
There is an error called TestBrawlError. Some of you might heard of this. I don't know what it is exactly but it has something to do with a Bar Brawl that is not working correctly. It fires tons and tons of errors and has been fixed in ErrorTrap, Awesome Mod and Overwatch. Those mods simply handle the error while EAs code bounces the game, making it faulty. EA will need to fix the code and make it work properly for that's not something the mods do. They simply ignore the interaction.
This error has been a major problem for the modders since (yes you guessed it) LN came out. This just might be the cause of many many errors you guys are seeing and this is way the game I tested was fine after I put Overwatch in (errortrap would have done the job as well).
So: EA, please fix that TestBrawl, BarBrawlError cause there is no way that you have not encountered that.
I will keep you guys updated on the twallan's answer concerning the stuck in Bed sims.
@caspinwall: I completely agree with you. The game does not clean-up properly. They create temporary items which never get clean properly. That same problem happens with Sims when the go on vacation sometimes leaving people with Clones of their Sims after the get back. Those extra cars are temporary items as well as objects stuck on elevators and invisible objects on lots. When I was doing to some testing back when LN came out, I changed a few community lots to different types (no visitors allowed to visitors allowed and back) and I always got messages that the game will need to delete some objects on the lots which I could not see. They were probably temporary musical instruments and books that the sims had left behind.
