The situation you described with Minnie/Mary definitely sheds some light on the situation, and helps me understand why you think your proposed model of polyamory would work for you, and her. As has been said on this forum over, and over there is no right way to be polyamorous, as long as it works for all parties involved.
Also i greatly respect that you feel that if Mary wasn't interested in someone, then you would lose interest. i hope that is the case in reality, as i know it's hard to just shut off interest in someone. It's obvious to me that you've thought long and hard about this, and i applaud you for finding this forum, and opening up to all of us.