Having known Mary for my whole life, I never thought of her as contrary. In the last years of her life she became fond of reciting limericks and rhymes which contained her name specifically, "Mary Mary quite contrary." Maybe, the sound of "Mary" spoken in her own voice and heard with her own ears added some solidity to her being as her being was slipping away.
I have no insight as to why sometimes I miss her deeply, and its totally OK at others. It’s none of the things you identified for yourself, like a possession. It’s not a place, like for Michael, not Sherry's intense photos, and definitely not strangers, like the silly chick from hospice.