The way I read it, Spouse met her when she had 2 partners (the couple). OP lived in her own home. Then, when OP split with the couple, she effectively became mono with Spouse and they built a traditional life together.
So in his mind, it must feel like when she got "serious" about him, she became mono. Now poly has become a need she has to act on now, their relationship must be less "serious" and joint life-building less of a priority.
My main point is that it doesn't seem like she's had other partners in their marital home because nesting came after the monogamy.
Good questions!
Spouse is safe. His mental health episodes were not violent. They were emotionally abusive. He is now stable, medicated, doing therapy and making a sincere effort to stay on track. He wants to stay on track for his own sake and that of his child, parents, and family. I don't feel it's unsafe to invite a partner into the home. If I felt he was unsafe I would not allow spouse to share a home with our child, either.
Looking back, it definitely would have been better to not invite Spouse to live in my home and us to make a home together. I thought I knew him well after 2 years of dating. We even went to a retreat early on with both the poly couple and ourselves, and we had dinner all of us together a couple of times.
We should have had those conversations about "yes we are mono now for the sake of the pregnancy/baby but there will be at time when I may not be mono anymore."
As a couple we are very compatible. There is a strong connection there even with all the mental health stuff that went on. In the thick of it he did move out for about 6 months to maintain the peace of the home and to for the child's sake-- as well as to give Spouse the room to work on his issues and healing. He moved back in about 6 months ago and has been making more progress.
Maybe this has to do with his fears of rocking the boat more than his fears of me loving someone else more than him and that kind of thing?
We had a wonderful talk yesterday after he had a good therapy session. He is open about his issues to this therapist, and he's also open about me being poly and our current issues. At some point we'll go as a couple for a session but he's not quite ready yet. The therapist pointed out this to him-- about him needing to have control. During one of his episodes he put cameras all over the place (hidden) because he thought I was going to cheat on him. He was having paranoid delusions and this was one of just several things he did-- but this one really affected me the most, obviously. He feels awful about this now. I have never ever cheated even in my mind or texting, etc. And he knew that, but he felt insecure about it anyway.
So, now that he's more aware of himself and his fears after the therapy, I think we can move forward with some compromises concerning poly Partner visiting our home from time to time. Poly partner knows about the mental health issues and feels comfortable with coming over into our home.
It's been great to think this through with the help of you all! Thanks! I'll update. We have a joint family dinner at a restaurant in about 10 days, Spouse, Partner, me, and child.