Must be a little confusing, knowing how far south things turned with Si and how much poly hurt Matt, and now to actually have him encouraging you to countenance a growing relationship with your best friend. I am sure you are terrified of making a mistake and messing up again.
I feel that you need some time to think about this, as well as some more time to discuss it with Matt. Let's make sure he's not thinking, "I knew she was going to force poly back down my throat again anyway; it might as well be with this best friend." We need to know that he truly feels good about her, trusts her, and is comfortable with poly in that context.
It would be a huge change, despite the tertiary status. Talking and thinking about it is already a big change, so that's what I recommend for quite awhile. Make sure to do lots more talking with the counselor about it too.
Don't get me wrong, she sounds like a wonderful person and a much better candidate for a poly arrangement than Si was. I just know how much you and Matt have both been hurt by poly (even if Si is the real culprit). We've got a long blog here of swearing up and down that poly is done, gone, finished, not to be considered ever again. Well I guess they say never say never, but, I officially advise you to be careful.
What a wonderful experience with your daughter in the cemetary. Almost sounded like "The Sixth Sense" for a minute there. I will just have to file it away in my mind and think about it for quite awhile.
Love means never having to say, "Put down that meat cleaver!"