SFR, geez, I know how you feel and I'm sorry. Perhaps sitting down at the very beginning of her stay and stating your boundaries right up front so everyone knows exactly how you feel and what you expect. For instance, being informed that she will not be home one night, so you don't worry. And, helping around the house, chores you expect her to assist with. The quiet you have become used to that she now has to abide by. Since she is clear she thinks she will move out in the fall, you might say, if things don't pan out by then, then you will insist at that time she enter therapy and take whatever medications necessary in order to continue living with you.
I would sit down with husband and write a list of what your expectations are and then make it clear to her. I would have an end date, like Sept. 15th, that's about 3 months,or whenever school begins. At the end of that time period, you will have to re-negotiate the terms. This is your home and you have all the power, she doesn't. She is now a guest in your home and needs to act accordingly. If you allow that guilt to run you, you will give your power away to her and you will be miserable. If it were me, I would look at it like a sort of opportunity for me to let go of my guilt and stay in my power and make sure I got my needs met. Make it about YOU, not her. Sometimes we have to go over old ground so we learn what we may not have learned before. This may simply be another step in your own detachment, so you can let go of the guilt and come September you may find yourself in a totally different space, happier, freer, without that guilt and able to REALLY enjoy your empty nest at that point. Sending you big hugs................this too shall pass!