This response is all predicated on this situation being real.
Ok. God, I hate to admit this. It's so embarassing, but I am going to have to agree with Mac on this one. lol.
She has completely lost respect for you, your wishes, and, more importantly, her wedding vows.
You may have started the ball rolling by getting her involved with Simon in the first place, but that really doesn't matter. What does matter is that you obviously did not set ground rules either with Stacy or with Simon. Both should have had certain rules that needed to be respected and followed and both should have known the consequences for breaking those rules. And at that point, if either broke the ground rules that were set out, you would have needed to follow through with whatever consequences were agreed upon. Since the rules were not set in the first place, you are kind of reaping what you have sewn.
The more important thing is the respect for you and the wedding vows. It takes a long time of honoring your word to keep respect, once the other party initially gives you respect. Once that is gone, most times, it never comes back. It can, but it takes a ton of work over a very long time to earn it back.
That is one of the reasons I am divorced. I went through a rough time, job-wise, of several years and she completely lost respect for me. Oh, she would say she never did, but you can feel it. It is all in the way they react to you and to the things you say. Once, they would have reacted this way to something you said, but now they are reacting this way. Why? What changed? Simple. They lost respect for you. And respect is one of the cornerstones of love, which means the love they feel for you, if they still love you, isn't the same. Even as she picked up the ******* and walked out on me, she still tried to convince me that we just needed a break apart to "work on the issues we each have". Translation - "You are really Fd up in the head. When you come around to my point of view and change yourself to match what I want and then get down on your knees and tell me how right I was and apologize all over the place, preferably in front of our friends, only then will I think about coming back to you. I probably won't, but I will at least deign to think about it for 5 seconds.". Yeah, I know. Bitter much? Maybe just a tad. I don't like being lied to. The point is that without respect, they will try to convince, or even try to fool, themselves and you that they still love and respect you. But you can feel the difference and it is not a good thing. Not healthy for either of you.
So. Where does this leave you? Unfortunately, IMHO, hiring a lawyer and fighting to keep whatever you have. You can try counseling, but I doubt she will go. Why should she? She has her cake and is eating it to and there is nothing you have done to indicate you will stop that arrangement.
If this is real, I feel sorry for you, but the over-sized lady with the great voice is warming up her vocal chords.