How is this situation any different than roommates who no longer get along? We frequently advise people whose roommates are making their lives a living hell that they can't just lock them out. We also note that if the roommate is a subleaser then they have a LL/Tenant relationship and proper notice to vacate and/or legal eviction procedures must be followed. It matters not that these two were "lovers" rather than just friends.
If you're suspicions that she'll damage your stuff are correct, you can get her out pretty darn quickly by filing a police report. If you really press the issue she'll likely even have a new, free place to spend the night.
I think OP's "thinking out loud" about whether the risk of his stuff getting trashed or the risk of her suing are greater is the proper thought process on this one.
We recently had a post about someone whose roommate had locked her out, and I think the consensus there was that the locked-out roommate first had to try going through the LL to gain access, and then could sue for any rent she paid during times she couldn't access the apartment (and I would leap, any of the locked-out roommate's stuff she couldn't recover from the premises). That sounds more likely than the police breaking down the door to let her back in.