Well, this has come as something of a shock, but I looked at the old version of the novel and I suddenly saw a way to make it work. Combined with a couple of days of writing an awful lot, I appear to have acquired a complete draft. It's a little shorter than I'd normally like, so maybe I'll need to add to it on the next pass, but other than that it's complete.
On a more general front, the whole episode raises some questions about me and the abandonment of novels. I have something of a history with it (weirdly enough often around the 28 000 word mark), so naturally I'm intrigued as to why it happens. General mental state is one thing. Caught in a sufficiently bad mood, the same piece that looks fixable can look awful. Boredom may be another, that point where the initial rush of excitement fades. At that point, something needs to be quite powerful to make you want to keep going. Going off too quickly can also lead to it, not just because problems that aren't dealt with early can really mess up the book, but also because you haven't decided yet whether the idea is a good enough one to commit to for the next few months.
With this one, it was probably a result of the nagging feeling that it wasn't good enough, combined with too strict a plan and structure. It took some of the fun out of writing, which meant things slowed down. When I went back to it and just did things the way that felt fun, it worked. I'm just glad that I kept this version around instead of deleting it.