You're happy with M, and A expects you to turn that on and off like a lightswitch. He left you, and now wants you to drop everything for him because he "may" have feelings for you again? Oh, gee, how lucky you are that he now "may" have feelings! What a schmuck.
I get that you miss A, but having a fond remembrance of what it used to be doesn't mean there is a basis for going back. It will never be the same. And your fond memories of how it was and who he was will fade in time. Plus you would be breaking M's heart.
I vote for sticking with the sure thing - your happiness with M. That is real in the here and now. Why throw it away? His ultimatum is unacceptable.