Desperate measures:
Success (only wasted my whole afternoon and most of my evening). I don’t know if this is what really fixed it, but it “appears” fixed:
I gave up, turned off the machine. Turned it on straight into Recovery Mode. Turned off SIP. Rebooted.
Lo and behold, it did not install Sonoma, and w/ SIP disabled I could delete all that /System/Volumes/Update crap. Someone is going to tell me “Oh, No! That stuff is needed…” Well, not so far, F’it.
Sorry for those who actually have to “roll back!” Yes, it can be done, but anyone who tells you it’s easy hasn’t tried it.
;~}