Afar, I notice two things. First, you said the marriage wasn't that stable prior to trying the open/poly approach. I think most of the other posters will agree that open and poly are not good ways to save a marriage. These are very advanced forms of relationship and not a bandage.
The other thing I noticed was the absence of a relationship between you and the other woman. You have met and conversed, but it didn't sound too constructive. Perhaps if you had a friendship with her based on more than just a man in common. You two could and probably should have something between you (not suggesting romantic or sexual) that is just between you. Him knowing that will really change things among you all. He won't know everything you do and say and he will have to respect that.
But it's probably too late for any of that. We all are sympathetic. You get a big hug from the forum members.