I wonder if you can quit MC, and just copy and replace the "view state (data).jmd" and "view state (index).jmd" files in the Library directory from a backup?
I bet this would have worked. But, it turned out when I looked, that the last auto-backup wasn't that long ago (on 3/20, late in the day). Most of my more recent file tagging was with Video files, and the JRSidecar.xml files would be able to save me on the library data for the stuff that had moved (and indeed, auto-import picked them up and they imported fine with all their metadata intact). So, I just restored from backup.
I lost the [Date Imported] metadata for all of those re-imported videos I tagged (all of these showed up as new imports), but that's no big deal, they were all newish anyway. I also lost the [Last Played] data for everything I watched since the backup date, which is more annoying, but I can live with it. I had to re-do all of the little View tweaks I did tonight, which was quite irritating (I was up to a lot, which is how I ended up renaming that view in the first place), but I got through it in about 15 minutes or so.
This little trap is quite dangerous though!
This needs fixed. Imagine if the last backup I'd had was older, or I'd done more complex tagging in the interim? That would have been terrible! The fact that you can rename a view to a "magic word" as simple and common as "Advanced" and have it just - POOF - vanish (and take it's sub-views along for the ride) is certainly not a good user feature! It's awful!