"I really don't think it has anything to do with 'possessiveness' on his part (that's not in his nature) -- it's just something he never in a million years dreamed he'd have to consider 'sharing' with other men."
Ah, but "sharing" and "possessiveness" are essentially opposite words. Maybe it's not normally in his nature to feel possessive, but maybe he has stumbled into one of the few things he does feel possessive over. If it were not so, then sharing this would by definition come easily for him.
Like jealousy, possessiveness often has to do with some kind of fear. Fear than M and/or T will be better lovers, fear that you'll leave him for them, fear that you'll spend more and more time with them and start neglecting him (not possible with the LDR's but fears aren't very rational and can conjure up all kinds of possibilities). Fear that his PIV with you will never be as good after M and/or T have had it. And so on.
"Needless to say the communication has become even more stressful for both of us, and sometimes has me wishing I hadn't opened this Pandora's box."
Sure, you might have left the box closed, wondered for the rest of your life what was in it, and probably built up some subconscious resentment over never feeling free to open the box.
Taking it all slow, and keeping the communication channels open with A, are your two most important strategies. Compromise when you can, but only with compromises that you can live with.
Hope the mono/poly Yahoo group helps. Also I can't remember if you guys ever tried seeing a poly-friendly counselor? something to think about.
P.S. Moving the thread to the blog section requires mod/admin intervention I believe. For that you would check out the Moderators and administrators page