Each time I've installed it, it has been for all users, and was installed using Administrator privileges
Didn't you say that you've installed Opera using a non-admin account?
I think I have confused the terminology--sorry. I have always installed it for all users using administrator privileges to run the installer program. But Opera itself has always been run as a standard user account, except for when I was running it in administrator mode trying to eliminate the UAC popups. The popups occurred regardless of whether it was being run as administrator or as a standard user. In this final test it is being run as a standard user, but was installed in the unique (non-default) folder. Prior to about October 15 it was being run as a standard user, installed in the default folder and was not causing the UAC popups.
I hope this clarifies the situation.