Interesting, though, that in this case the Upperworld *is* the natural home of the Beast and, if Abigail isn't meant to be frozen on ice, then at least the Underworld of the planet below is where she belongs. But the two *are* disconnected and the world is disrupted because the energy (song) can't flow between them

You're right and it does link back to that idea of the duality being an illusion, perhaps. That it's not about who belongs where but that the connection between spaces isn't broken and that thresholds are able to be crossed. Perhaps like you say, it's not that they're in the worng place, only that they are held there and not allowed to transit freely. Maybe it isn't about angel/beast, male/female, above/below dichotomies, but is actually turning that idea on its head, showing that separation is an illusion. The angel can belong in the underworld, the beast in the upperworld because at the end of the day they are the same thing, connected. Ooooh.

It actually first occurred to me that the shark was River too by the most ridiculous chain of logic
lol, i like your chain of logic there...but it's these small things that we have to look out for. I love it when i notice a small thing then my head goes in a really weird direction, then suddenly i'm having a revelation about some symbolism or whatnot used in the show.

Lines about being eaten/the threat of eating seem to come up a lot in Moff
I know, we did an analysis of series 5 and 6 on gallifreybase, and seriously the number of mouths - usually monstrous mouths - loads. I should do meta on it, but... *vague hands*