This is resolved, though the underlying problem isn’t. I was able to get things working again by restoring randomly between database and file back-ups from various dates. Eventually I got luck and found one that works. Somehow I updated the database and have 4.2.1 working, with 12 updates to do, but the site is working. I”ll step through these updates slowly and see if anything comes to the surface.
Marking this as resolved for now.