Selkie,
Wow, you have a lot going on. Your son coming home for a short stay and turning out to be long term. All of his problems, illness, and your other son's wife. That is a lot!
You are on the right track in working out a plan for him to move out. I had something similar a few years ago, my therapist worked with me on a plan for my daughter to move out. I was worried that I would not be able to evict her, as I was being coached to do. Fortunately, she was accepted into a transitional housing program. She was not receptive to living on her own, as she hates to be alone. But, she turned out to be okay. And, actually, says that at times,she enjoys being alone.
I am glad that you are in therapy for yourself. It really does help.
You seem to have insight on BPD. When was your son diagnosed?
Please come back and post. We really have a supportive site here. It does get better!
peaceplease