open up regedit and goto "HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Uninstall". You should see subkeys for each of those versions. You can right-click the Opera keys and delete them to get rid of them from the list
Did it, and after a restart, the only Opera entry in Apps & Features is now a good red-circle icon and a version number of 84.0.4316.19.
Opera still doesn't work in my user account. Probably works in the admin one, but I didn't check.
Editing the registry just now reminds me I hadn't done it in 15 years at least. Used to be, in my experience anyway, some uninstalls didn't get completely wiped off the computer, so I'd enter a keyword (as in this case, Opera) and search through the entire registry. F3 would bring up the next result. Sometimes it would take a handful of F3s, sometimes dozens.
I appreciate the interest and time you've taken in trying to help me.