It seems to me if he was considering staying with B, that he might expect to get the whole package deal with her.
Another thought on B: do you know her personally? Did you know her personally before you started hearing these things about her from him?
The reason I ask is that, your story makes me wonder how upfront he was being with her about his relationship with you. She came to his home and found texts between the two of you. Was she open to poly? Was she poly? Did she enter this relationship knowing and expecting that he'd be seeing other people? Or did she start to get an inkling that something was going on with him and someone else and he wasn't telling her?
Because discovering that someone you thought was your boyfriend, father to your children, and committed to you and only you is actually involved with someone else...well, for short let's call it cheating...DOES raise some very powerful emotions. Fear, abandonment, betrayal, much more.
I've been in her shoes, and I imagine that virtually every one of us betrayed spouses on the infidelity forum has been called 'abusive' by those who a) are as lacking in common decency as the cheater and/or b) have no idea of the whole truth of the story.
I'm not saying she's not abusive. I have no idea. I'm not there. But I'm asking for more information, because I've seen all too often in my life how a person can be unjustly brushed with accusations that look very different when the whole story comes out.