I'm enjoying reading your story and rooting for you guys.
Quick question: if it does work out between M and his friend, have you considered STILL giving the triad a shot? After all there isn't a number limit with polyamory
If you'd be comfortable with M also having that relationship, if M feels able to juggle three relationship, and if M's friend is fine with it, it could also work out.
Might be a lot of "if"s, but I was wondering if you had considered it. Polyamory often means you don't have to choose between two or more relationships, if everyone is on board, and that is also the case here.