OK, fine, very quick update: I've met and interacted a fair amount with the younger daughter; it's a nontrivial proposition getting her accustomed to what's going on, but we're well on the way. It certainly helps matters that she's very cool, and it's absolutely obvious that we'll eventually be getting along well — in fact, this morning C [let's call her that for now, though actually tentatively chosen a POG name for her, which I'll put up after I make the account] said she was confident that one day I would walk [daughter] down the aisle.
And from that you might conclude the magic continues; it does. We're up in NH now, acquainting her a bit with my world and some of my family. And everything is still ... absolutely, indescribably right.
And yes, she now knows about POG, and is reading this thread — and she thinks it's wonderful (and likes some of the posts about this whole insane set of events very much — she's well equipped with a sense of humor, too).
Much more by way of updates down the road, but tbh I have other things to do now...