Wow, what a mess. I cannot argue with your right to not want to finance what you consider an immoral living circumstance between your daughter and her bf.
I'm not going to pass judgement on you for mistakes made in the past, because I've made them too. There are many here on this site that have never made a mistake, and they have a good time rubbing everyone else's nose in it.
You really have to consider that because you left, you were not able to guide your daughter down a different road than which you and her mother walked. These days it doesn't make a difference in some families whether both parents are present in the home, or not. Children know their "rights".
You aren't under a legal obligation to provide college tuition for your daughter unless it is spelled out in the custody and divorce decree. I would rethink downsizing the health insurance, though. At least for now.
Face it. If you have talked to your daughter about your views on "shacking up", and she's done it despite your objections, then threatens to sue you....you don't have a daughter, you have a programmed leech. I wouldn't put up with it.
Hope things work out for everyone.