Ray,
As you say, the issue with HDMI connections in older MBPs doesn't seem to be the same as that with the rMBPs, although it does all seem to be related to 10.8. I notice you have 10.8.3, and 10.8.4 is now available, not that I'm confident it will help to upgrade. Is there any firmware upgrade you may have missed along the way?
That said, one problem some people had was that they didn't get rid of /all/ the plist files, and not everyone had the same number of them in the same places on the hard drive. They could appear under the root library preferences, user library preferences, and also the User/Library/Preferences/ByHost directory. And some of them are com.apple.windowserver.xxxxxxxx.plist where the x's are numbers. It goes without saying that you have to reboot the computer and preferably do a reset with option-command-r-p on reboot. And it might be a good idea to have an HDMI cable connected to something during the clean reboot. The windowserver plist files reappear when you reboot, but in the rMBPs they are the right ones after the HDMI is working again.
I'm probably not telling you anything you didn't know already, and with the non-rMBP this may not work.
Other people have had cable and adapter problems, as well, although the Belking one should be good.
Good luck with the Apple Genius. There are varying degrees of genius, as you know.
Ken