Well, that seems to have taken care of it. 2nd day and it’s still running strong.
I can conclude that it’s not PC specific or hardware related, because it did the exact same thing on my work PC (8 month old HP desktop, Roaming folder on secondary HDD) the very next day after it crashed out on my home PC (2 year old Toshiba laptop, Roaming folder on primary SSD) - 4 crashes with 3 automatic restart attempts. The only thing they have in common is the version of Opera installed and the version of Windows they’re running. I repeated the same clean/reinstall steps on my work PC, with no changes to ownership/permissions of the profile folder, and it’s working fine now, too.
Thank God, because 1) I was dreading the possibility of having to migrate back to PaleMoon, and 2) after dealing with other people’s PC problems all day, I REALLY don’t like having to mess with mine.
OK, I'm up to 2 hours without a crash on this 2nd repair attempt.
Temp profile was not needed this time. Saved a copy of the Roaming "opera software" folder, uninstalled Opera normally, deleted both Local and Roaming 'Opera Software' folders, deleted the Opera folder left in Program Files, then ccleaned the registry - it removed a lot of dead stuff with Opera's name on it.