So now I know what I can do with the middle of the book, which was somewhat vague in my mind. Roughly. I don't have details of scenes, just the outlines of them.
Saturday, I trimmed my slow opening paragraph, then set to work with my synopsis and chapter outline; I had to fit one scene in fairly early, so my new idea could be stuck onto the end of the current draft. That one scene re-interprets the subsequent scenes. I ended up with a net gain of 327 words, and that was it for the weekend - but now I am letting my backbrain chew on it, so I can get back to work on Saturday morning.
I have too many subsidary characters (as usual) and will no doubt have to collapse some of them into each other. The characters sit around and talk too much. I haven't taken full advantage of my setting. But the thing is not as bad as a I neurotically imagined at this stage, which means everything.