Updating to say - I took the derailleur off again, pulled off the c-clip and removed the body screw plate and seal. The spring inside wasn't going anywhere even though I pulled on it hard, nor was the bolt. Full of salt and corroded gunk though. I guess that's what winter riding does to a bike. After a lot (a lot!) of oil and spraying and squeaking and working back and forth until the whole thing was pretty warm, the bolt deigned to wiggle slowly out. It was a close fit, and the inside of the hole was just visibly plated. Scraped it out, trying not to make gouges, greased it up well and lo and behold, the bolt went in again and could be turned by hand.

Reinstalled I could see the derailleur pivoting a little around the bolt when I re-adjusted the gears, so I'm hoping this will solve the mysterious sticking problems back there. I still have the sloppy pulley wheels to replace, but I think they just cause imprecision.

I like taking things apart