You could always try artificially creating that registry entry so that it can be deleted!

Yeh, I'd try that. In "Software\Microsoft\Windows NT\CurrentVersion\AppCompatFlags\Compatibility Assistant\Persisted", right-click and add a new dword with its name being the path to the opera installer exe and its value being 1.

Other things you can do:

Download the 12.16 x64 installer, launch it, click "options", set "install path" to the existing location of your 12.16 x64 so it says "upgrade" and install. That should fix the registry settings. But, if not, try downloading and installing the 32-bit version of 12.16 and install that to "C:\Program Files (x86)\Opera".

Then, do the same with the 12.17 installer if you have to.