Those are a lot of questions. You might want to do a tag search for "secondaries" (secondary merged thread comes to mind.. I can't remember its full name) "triads" and anything else that you can think of. You can do a tag search through the search engine.
You have no rights in marriage to this man if he is already legally married. I don't know where you live, but I can't think of any country that provides this kind of "protection" (What do you mean when you say that anyway?). That doesn't mean that countries don't exist though.
It sounds like your best bet is to keep at creating a relationship with her of friendship. There is no need to create a triad, but metamours getting along is pretty much essential. When she lies and manipulates, ask her what her intent is and call her on it by telling her how it makes you feel.
Really, the two of them are going to have to work it out for the most part. Keep providing them with your boundaries and listen to theirs until you come up with some agreement on how it will work. It all takes time and effort. Neither of you are entitled to him so get about occupying yourself with your own life and do your best to make it so he doesn't have to be the mediator between you and his wife. These would be my suggestions anyway.
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