Well, I can't say I identified the cause of the problem, but I was able to resolve it.

I deleted all three partitions on my OS drive (via Paragon, but this can also be done with the Windows Install disk, IIRC) and ran the restore from the backed-up drive image one more time. Worked. Have rebooted twice since and things appear to be once again back to normal... until the next time. Haven't identified the common denominator for a cause yet, but this last one was immediately following a failed Windows Update, which I understand can do bad things to boot managers. Guess we'll see.

Kinda wish I'd have thought of this (it was suggested by a friend) before I replaced my original OS SSD, except for the fact that this new one's write speed is ~50% quicker than the read speed on my original drives. Not exactly a bad thing.

Thanks for the suggestions. I plan to leave the Secure Boot set to "Other OS" in case that update comes into play, since things boot fine as-is.