Very true. One thing I would love to read more of is John fic. Early years, when the boys were still very young, and he was newly alone and learning the life of the hunter.
So it's very possible that all of these people that knew John, may have known Mary in her younger days. Or at least known of her since Grampa Campbell liked to work alone. Do you think Kripke planned this working so well, or was it dumb luck? Because I wouldn't have put us here three years ago, but this episode certainly supports where we are.