I'm glad we got the other side of the story! It sound to me as though you are truly sorry for what happened, and are really trying to make it work. I don't know that my post will help any, but just to give you another perspective:
I mentioned a woman named S on here earlier. She's my fiance's ex. They were together for 5 years, and raised HMA's son together. They were really building a life. A lot of things were wrong with their relationship when I came along - and I was only a friend to him. I was in a bad relationship too, so we were able to commiserate. The mutual like of one another as friends, combined with he and I being one another's "type" eventually, well after S left, got us into a WONDERFUL relationship.
At the beginning, there were a lot of problems. I had to spend time with S. I had to watch HMA - who I was in love with, agonize over her or me, her or me - because she couldn't be in his life if I was there. Not even as a girlfriend - she couldn't handle me as a friend.
Later, there was talk of a three-way relationship. A triad, or a V, depending on where things went. I went along with it for one reason only. Because I knew S would NEVER let it happen. And I'm really glad she didn't - because if HMA had come to me and said, "I know the circumstances surrounding S, and the relationship you may or may not have with her are very negative. You don't like her, she doesn't really like you, whatever. But I can't pick. I'll be with you, and she can tolerate me being with you - but I'm going to be with her too." I can say beyond a shadow of a doubt that I would have left. I would have been there for HMA, been a friend - but I couldn't have been in a relationship with them, not even a V. Because of how little I respect S, and how much I dislike her, and how horrible the situation was that involved all three of us.
I would never force HMA to cut S out of his life - she has his kid for now, one state away, and they're learning to talk as friends. If a relationship ever started to re-kindle there, I would put my foot down, just like TheMountainBoy did to you. I would tell HMA he could either be happy with the fact that I could handle them being friends, and respect that I can't take it, and that they broke up for a reason - or I was out. Gone. I couldn't do that to myself. There comes a point where personal happiness is more important.
When I was going through my serious trouble with Anne (our current third) and struggling with our relationship, there was always the "out" for me. A veto power. HMA wasn't willing to compromise what he had with me, and the other possibilities out there, for one girl. He was in love with Anne, yes - but our relationship, and the possibility of other relationships with a different girl existing was more important. His willingness to help me, or let it go for me and for the relationship HE AND I had was what ultimately drive me forward. I'm glad he did, but that tactic wouldn't work in every situation.
Frankly, Jools, if I was TheMountainBoy in your situation, I would be bracing myself to leave you. He's been able to handle you being with other men, he's been able to continue on with poly despite getting burned by cheating. I don't know that I would be that strong, so MAJOR props to him! You have access to ANY OTHER MAN on the PLANET that will have you, TheMountainBoy has only asked that because of the pain that was caused by your cheating that you not develop a romantic relationship with M. That's what I asked of HMA. We had an open relationship, yes. And I was so uncomfortable with S because of what happened, I asked him to not fuck her. ANY woman but her. He agreed. It wasn't that much to ask.
You need to very, VERY carefully evaluate whether your relationship with M is more important than the one you have with TheMountainBoy. Because if he can't handle it - some people just can't, and I'm one of them, you're going to lose him. Is it worth it? Or is it just you rebelling against a "rule"? Is it because he's the one person you've been asked NOT to have, there's now an incurable allure to the forbidden? Or is your love for M THAT strong that you HAVE to have him, all consequences be damned?
Talk to TheMountainBoy about it. If he can't see it working, and he can't budge on it because of a moral issue or whatever - if he can't handle you being with M, evaluate closely. Don't begrudge him his feelings, they're valid. If they can't be worked through - you have a decision to make. The ball will be in your court. Pursue a relationship with M at the expense of the one with TheMountainBoy, or lose TheMountainBoy and move forward with M, and be poly with him. Seek new opportunities and new lovers without TheMountainBoy in the picture.
Just my 2 cents, and how I feel. I'm more than happy to PM you about it, if you need a sounding board or a differing opinion.
In the end, we're all here for each other!
(I hope I got the names right. Correct me if I'm wrong!)