If you can use System Restore see what restore point dates are available first. If you didn't have the problem with UAC at a previous point then try a restore to that point.
System Restore keeps your system as it is but rolls back updates or changes for system level items. If you installed software, made system changes or got a windows update then these will be removed when you restore back to a previous date. (doesn't affect personal files)
As I've never tried it after adding a password to an account I don't know if your adding a password will be removed but if it completes successfully try to sign-in using your new password first, if not accepted try it blank.
As I see you are following another users suggestions then finish with those first before the above.