So I had to delete the last 3rd of the book. I now have the new last 3rd outlined and I dictated three chapters today, so not too shabby. I won't increment the word count bar until I get the transcriptions back, though. I expect they'll require significant editing and rewriting. But still, it feels good to be going forward again, rather than in the other direction.
Let me use this opportunity to say how much I adore OmniOutliner!