Well, I hazarded a reinstall, and it fixed my Red October problem; it's working again just like before. Not sure why that should be the case, but my original problem is fixed. It just created another one for me.
I told it not to remove registry entries in the hopes that my settings would be preserved. However it reverted my settings to my settings from around three weeks ago? Not sure why exactly it would revert to settings from that specific moment in time, but about three weeks worth of trial and error with DSP (and paddling around) is now lost, including all my zones/zoneswitch rules :-(
It would be really handy to have a way to create a snapshot of one's settings, or otherwise be able to copy them off somewhere (unless there's already a way to do that, and I missed it). I've been working with a really fiddly biamped set-up with homemade speakers and I'd love to be able to save a "last known working configuration." As it is, I think I'm just going to have to write everything down in case I ever have to reinstall again :-( If there are going to be periodic reinstalls to get things working, I'm fine with that, but it'd be very nice to have a reliable way to store settings.
To be clear, this is the first actual problem I've had with J River in a few months of fairly intense use (and endless fiddling with settings), and J river does things most other software can't even touch (much less hardware). Overall I'm a very happy camper, and I'd like to express my appreciation to the devs for a really amazing piece of software, this hiccup notwithstanding.