As you have experienced, the philosophy of "pretending poly is okay" (fake it til you make it) doesn't always work. Maybe it works in some cases where one spouse is overly dependent on the other spouse. It's definitely not a "one size fits all" solution.
As you are already kind of doing, I would be very cautious about how much association you have with Si, but a cordial amount of association might be okay. There are serious trust issues there given the fact that Si admitted she had wanted to eventually nudge Matt out of the picture. You have a tough judgment call to make as to whether you think Si can turn over a new leaf. I'd take it real, real slowly, but it looks like you're already doing that.
I'm glad things are currently going pretty well for you, Matt, and the kids. May the relationships between all of you continue to flourish.
Love means never having to say, "Put down that meat cleaver!"