Caster never reached the Temple. As she lay dying, in another time and another place, a small boy collapses as a circle activates that shouldn't exist. Just before the end, Caster is summoned into a world that seems like a mockery of everything she has ever known. But to ten year old Harry, even the Witch of Betrayal is salvation. It certainly can't be worse.
And what, I can write up contracts that can strip the magic of any of my descendants that are inescapable? So how about one that says 'Don't be a bad person? or even 'Don't marry muggles? I can't see certain families passing that one up. And what is a marriage, anyway. If it is so arbitrary, you expect me to believe 'magic itself' or something cares about human marriage?...