I am fine with you and Matt and the decisions both of you have made. I think Matt has gave the impression that he is an angry person, when the truth is that a decade of passive tolerance finally blew the lid off the pressure cooker. I think he vents on Polyamory.com, and some folks misunderstand.
Sometimes, the nicest people in the world have to draw a line in the sand. My oldest brother is the most peaceful, loyal person I know; wouldn't hurt a fly. But one of his loyalties is to the church, and that means monogamy is a principle of honor in his mind.
So, when his first wife had an affair on him (and got caught), he took it very seriously. He was willing to try to patch up the marriage until she said, "I'm going to start seeing the other guy again. Deal with it." He dealt with it by divorcing her.
Mind you, she had treated him terribly long before the affair. Much verbal abuse, exploiting his goodwill, and resenting his good relationship with his brothers. He and she lived with my wife and I for a few months, and I remember being awakened at 3:00 a.m. to the sound of her, downstairs, speaking harshly to my brother, when he had to be up at 5:00 a.m. to go to work. She didn't work and would sleep in til 2:00 in the afternoon.
But only when his moral principles were violated would he finally bring that frightful marriage to an end. And once it did end, he was quiet but firm about keeping it that way. He severed all contact with her. Years later she wrote him a letter from the depths of humility, admitting she had treated him wrongly and begging him to take her back. He didn't answer her letter.
I'm glad you and Matt are rediscovering your romance with each other, and I hope Si happily finds her way in life, but see no reason why you have to be involved in that. The important thing now is removing that wall Matt has up, one brick at a time as you said.
I'm sure you'll do fine. Enjoy the rest of your vacation.
Love means never having to say, "Put down that meat cleaver!"