Sounds like Matt's wisdom was largely the reason why you didn't end up going to visit the U.K. by yourself. I can't imagine carrying the burden of so much death (and pending death) all at once. It's good that you've had a support system by your side.
I would say that you are fortunate in inheriting "the right genes." My ancestors didn't leave me great genetic material to work with. My father's side of the family is clueless and my mother's side of the family is nuts. I've been on medication for many a year and I'm still stressed out most of the time.
With all that's on your mind these days, I'd recommend approaching the best-friend situation with unhurried caution. There are so many riddles to solve about Si, poly, and how it all affected Matt. Remember many of Matt's walls are still up. You need more information about right where he stands, and how to do right by him in the future.
Sometimes it helps to just try to focus on what you do have in life (when there's so much you've lost or are losing). Some things can't be fixed per se, they just have to be accepted and acknowledged.
Good luck with the rest of your trip.
Love means never having to say, "Put down that meat cleaver!"