What happened before the Little Mermaid? Where did the Sea Witch come from? And what happens, if history repeats itself?

The middle dragged on a bit, but the ending was gripping and twisty, just as I love it. For a long time I felt like there wasn't really a problem to be solved there, just a side-plot turning into a main-plot for lack of something more interesting to happen. But the last 100 or so pages really upped the game.