It was just a surprise to realize who was who here. I think you are being quite magnanimous in attempting polyamory after experiencing such a betrayal. You don't have to accept his mistress into your life and marriage if that is not what you want or feel comfortable with. It's my opinion that your husband should try to refocus on his marriage and repair the damage he did before continuing any extramarital relationships.
BTW, I didn't suggest a debate with Crucnchymama, but quite often when two or several people are involved in a relationship and everyone posts here, a productive conversation comes out of it - if only because now everyone here knows who they're talking to!