You definitely sound like you subscribe to an "ownership" relationship paradigm, and that's fine for some folks if all (or in your case, both) involved are on-board with that sort of thing. But because of that, i don't see ANY outside involvement on your husband's part working out for you long-term. I see three possible outcomes when it comes to your marriage:
john and sarah break up now
You and john break up now
You and john break up later
I don't see any type of compromise or middle-ground working for you and john over the long-term. Basically, he needs to choose you OR her.
Not judging any of you. It sounds like you're all good people who want to do what is right. But this is a case of "can't please all of the people all of the time" or however that saying goes. Someone is going to come out of this with a broken heart (besides Mike, to whom the damage seems to have been done).
I am sorry i cannot predict a happy ending for all involved. It sucks big-time, but there it is.