I think right now, you are of the frame of mind that you don't object in theory to having an additional romantic relationship; however, in practice you have decided that sticking to just one romantic relationship is what you want, and the right thing to do. It is of great importance that a person's time and energy are finite (even while a person's capacity to love is infinite). Sometimes poly people (poly in theory, by "orientation" or "identification," or poly in practice, poly by number of partners and life situation) forget how important it is that one's time and energy have limits. No matter how many partners someone has, there is always a point where you'd have to say, "Shew! I have too many partners. I can't juggle it all." Sometimes two partners is too many. That's the situation you've found yourself in (and I respect that).
Sometimes I think poly people dislike the idea of a "poly community" because the idea can be abused and the community can become like a club, where (in addition to knowing the secret handshake, ahem) you "don't belong" if you don't "fit the definition of poly." I abhor that misuse of the word "community." The poly community I want us to have is one in which anyone who wants to belong -- who needs to belong -- can. Of course I'm in favor of barring people who only want to "join" in order to conduct a spam campaign, but that's quite off-topic from your situation.
If we need you to "qualify as poly," I think we can easily do so because you spent so many years living in a poly situation (the best way you knew how at the time), and because your heart and mind are open to poly as a concept. But I'd rather we didn't require you to "qualify." You have been with us on this site for some time, and posted many posts, and I for one have taken a liking to you and would not want to see you go -- least of all because you didn't feel accepted here or because you felt that you didn't qualify to "wear the club label." I am sorry to hear that some of the people you know locally have given you a bad time about your decisions. Your decisions seem sound enough to me.
Personally, I think your story is of great value to this forum (besides your value here as a person). You have experienced a situation in which a poly life needs to be set aside, and you have articulated the reasons for that change. I think it is important for people to know what they're getting into when they choose a poly life, and what they may be giving up. You have gave us a valuable example of that information. I think many people will be helped in their personal lives because of reading this thread. Even if they choose to have two or more partners, they'll at least be advised of some of the pitfalls.
I also find it a relevant topic to post: "What happens if you have to back to a monogamous life?" That is a question that many polyamorists have had to face, and many more will have to face. This thread lets them know that they're not alone, and gives them valuable food for thought.
So, by just about any measurement, I find your presence here valid and important, and I hope you will feel that you have some freedom to speak what is on your mind, and what is going on in your life. To me you will always "qualify as a polyamorist," because it will always fill a big part of the book of your life. But sometimes a person can be polyamorous and monogamous as well. They can be polyamorous in that their mind is open to that way of living, and simultaneously monogamous because they happen to be sharing their current life with just one partner. No need to wear either label, but you could legitimately wear both if you wanted, as far as I'm concerned.
In general, I just want to thank you for starting this thread and sticking to this thread. I know you have a lot on your plate and the time you take to write here is time taken out of your limited schedule of your own free will. I even want to say thank you to Matt for not begrudging you the time to write here. Reading this thread has helped me. It has given me a lot of food for thought, and put polyamory in a new light and perspective for me.
I look forward to your future posts, and wish you well in the very big move you're scheduled to take across the globe. You will be building a whole new life from scratch. All the people you knew locally will be far away. I am lucky to be able to look forward to continued association with you, even though we do live and will live far away from each other. That's part of the miracle of the internet, but the miracle only works if people are willing and able to work with each other, and welcome each other.
I feel bad for the pain all three of you have been through: you, Matt, and Si (and even the kids have been affected by it). I think all three of you have had your own ways of trying to cope with that pain. Some people would take issue with one of you and say, "That's not the way to deal with it!" But I think we'd all be dysfunctional in our various ways when faced with that kind of pressure. Si's reaction has been to withdraw and pull away. It is within her rights to make that choice, after all it is her life. The only thing I wish she'd do is explain things to the kids a little. But now just isn't a time when she has it in her heart to do that, so I won't complain. At the same time, it does put you in a bad position where you have to answer the kids' questions that Si isn't willing to answer. You just answer as best you can, and move on with life.
Don't mean to hog the space on your thread, just wanted to let you know that you have my vote of support.
Love means never having to say, "Put down that meat cleaver!"