For testing, you could delete Opera-related keys/strings one by one while testing after each delete to see if you can narrow it down. I'd start with the StartMenuInternet ones.
HKEY_LOCAL_MACHINE\SOFTWARE\Opera Software HKEY_LOCAL_MACHINE\SOFTWARE\WOW6432Node\Opera Software HKEY_LOCAL_MACHINE\SOFTWARE\Clients\StartMenuInternet\OperaStable HKEY_LOCAL_MACHINE\SOFTWARE\WOW6432Node\Clients\StartMenuInternet\OperaStable HKEY_LOCAL_MACHINE\SOFTWARE\WOW6432Node\Microsoft\Windows\CurrentVersion\Uninstall\Opera 54.0.2952.60 Opera Stable string in "HKEY_LOCAL_MACHINE\SOFTWARE\RegisteredApplications" HKEY_CURRENT_USER\Software\Opera Software Opera Stable string (if it's there) in "HKEY_CURRENT_USER\Software\RegisteredApplications" Opera Stable key (if it's there) in "HKEY_CURRENT_USER\Software\Clients\StartMenuInternet" HKEY_CLASSES_ROOT\OperaStable HKEY_CURRENT_USER\Software\Classes\OperaStable HKEY_LOCAL_MACHINE\SOFTWARE\Classes\OperaStable
(There could be multiple keys that when deleted trigger a fix (because the deletion breaks an association), but it's worth a try.)
I'd also look at "HKEY_CLASSES_ROOT" and check the extension keys for any extension that might be handled by Excel/Office (like .xls/.doc etc.) and check their OpenWithProgram IDs or Persistent handler IDs to see if Opera (or its ID (do a search for the id)) is listed there. I'd also look for excel-related keeys in "HKEY_CLASSES_ROOT".
It will take a while to test, but i think it's worth the try. I will keep you in touch of the progress