I was in the same boat for years on the serial monogamy, wish I had known about poly thing... and then I found out, and sorted things through, and found people to be with, and life is great now.
I see potential red flag waving in the breeze in your post:
"T" is extremely protective of her space and life, and has told me that if I were to ever have a girlfriend, she wouldn't want to have to see her or deal with her in everyday life at all, and she wouldn't want my relationship with the other person to interfere with my relationship and family.
It sounds to me like T wants you to have a mistress, not another primary. Is that what you want? It's swell if it is, but if it isn't you and T have a lot of talking to do before you can take any romantic/sexual steps toward M.
Has T met M? Does she want to? Do you want her to? The long distance makes it difficult, but I can't rate highly enough getting everyone in the same room-- not to have The Talk, necessarily, but at least to have dinner and swap stories. It sounds like M's situation makes her getting out to visit you both pretty difficult, but maybe you could both visit her, after
you all talk and feel ready for that step.
Thanks for sharing, and keep posting-- we're here for you.