Hi there Mountain Boy! You aren’t wasting anybody’s’ time. You are looking for feedback and answers. There is NOTHING wrong with that.
I can see why you are reluctant to accept “M” re-entering your life. Clearly his involvement with your fiancé doesn’t hold the fondest of memories for you. I think that’s understandable when your first encounter with him was one of deceit and mistrust.
You’ve clearly opened your mind up to polyamory, as you seem to be okay with “R” being in your lives. It appears that it’s only “M” you have an issue with (understandably).
Okay, first and foremost, you need to let go of your hate for “M.” It’s only going to eat you up inside. Instead, work toward being willing (and able) to talk about how you feel about him and your fiancé and WHY you’re uncomfortable with him re-entering your lives (and what, if anything, might make you more open to allowing him back into your lives). That’s more productive, and it won’t give you an ulcer.
I think issuing ultimatums is the absolute worst way to obtain what you both want (you saying you’d leave her if she saw “M” again, and her pulling the “if you really love me, you’d let me do what I want” card). Nor do I think that her declaring that she “is eventually going to see him again” as beneficial to YOUR relationship (particularly knowing how you feel about it).
She needs to understand that if she really loves and cares about you, she needs to take your feelings into consideration. And you need to understand that, if she really does love this guy, she can’t just turn those feelings off. Perhaps TRYING to ease back into things with “M” might be the way to go. Maybe you can agree that he can try to SLOWLY re-enter your lives (example: having dinner, TALKING among the three of you, etc.). Once you become more comfortable (IF that happens), then their relationship can possibly go to the next level. Perhaps you could try to give this guy another chance if he really does mean that much to your fiancé. But you all need to work together on this. She can’t just go off and do whatever (or whomever) she wants, completely disregarding your feelings. And “M” needs to understand and respect YOUR position in your relationship with your fiancé and respect the boundaries. I think the three of you sitting down and having MULTIPLE discussions, trying to get to know each other better, figuring out what you all want from this, and also expressing your frustration and mistrust (as well as working on solutions for this) is the only way everybody MIGHT be happy. (At least it’s a positive aspect that your fiancé has said that they’ll be honest with you this time around.)
Just my two cents.
As a sidenote, I don’t think she’s really being fair to “R” if she is focused on eventually reconnecting with “M”. But I could be misreading this.
Good luck! (And BIG HUGZ...sounds like you need 'em!)