Hi, reader50 (and others). Apologies for this late reply; I thought I had posted it 4 days ago but apparently not ...
I tried that (booting from the two problematic volumes) just in case; both of the volumes with 15.7.3 installed still failed to boot. Weirdly, I noted that both installs asked for passwords, sometimes three or four times, when logging in or accessing the Startup Disk control panel, on both the Sequoia and the Tahoe volumes. I would have thought this a Keychain Access plist problem, or something of the kind, if it weren't for the fact that it happened on both volumes (Sequoia and Tahoe). Also and equally weirdly, if I just hit 'Cancel' each time ... they let me in anyway, though they DID require a password to actually change the startup disk.
At this point, I have erased, formatted and installed Sequoia 15.7.3 on the Apple SSD and am in the process of setting up apps, etc., and moving user data to that volume. It has been rock-solid through this whole adventure.
Though I love a puzzle, this was just taking too much time and energy, both of which are desperately needed elsewhere. My current plan is to finish getting everything set up on the Apple SSD and then clone that volume to one of the 990 Pro SSDs; that way I have a stable system to fall back on while playing with the 990 Pro install. It also resolves any formatting/OS install issues. While on the 990 Pro volume, I'll save all new or updated user documents to the Apple SSD and will back up that entire (Apple SSD) volume, using Time Machine, to an external SSD. I'll post any additional results, including an "OK" posting if all proves well, in case any of this is useful to others.
Thanks for all of your help, reader50, mattspace, BigWaff and tsialex for your help and advice; many thanks, also, to all those who spent time and energy reading this thread.
Appearance - I have to say that, though I don't mind the liquid glass appearance on my phone, I'm not fond of it in MacOS, and neither Tahoe's overall "blankness" in appearance nor its icon-coloration choices are particularly friendly or attractive. I remember Copeland fondly; it allowed users to control the UI appearance and there were several user-developed themes that were both attractive and imaginative. "Paper" was my favorite; it gave you a drafting paper-like appearance. Another stroll down memory lane ...
