Final post-script, I'm sure...
Heard back from the author of Driver
Fusion regarding those three Registry keys which apparently couldn't be deleted. He advised that I need to check the "FORCE to delete entries" in the "Cleaning" section of Settings, which for some reason is not on by default when you install the product. He also said that while it appears to still be present after the "clean" (and they currently don't have a way to avoid that but are working on one), when you re-boot you will see that the three entries are indeed gone.
He then said the 12.6 install should work. It didn't. Same failure of driver to install.
He also suggested (based on his own experience with a laptop that also has a legacy ATI card in it, so that he too has had to fight to get the 12.6 drivers
installed) that if a straightforward INSTALL doesn't get the drivers installed, that after that I could simply try the normal Windows approach to updating a driver... pointing to the folder into which the ATI 12.6 installer file expanded so that the relevant INF file(s) would be there.
In other words, Device Manager -> select ATI 4800 Series Display Adapter -> Update Driver -> Browse my computer for driver software (manual install) -> specify the folder in which the ATI 12.6 installer got expanded and check "include subfolders".
So I then tried that "plan B", and amazingly the Windows driver update process apparently found the proper INF file and proceeded to actually install that 12.6 driver!!! I was astonished.
Re-boot, and sure enough it would appear 12.6 was now installed and operational!!
Except that right-click on the desktop did NOT show Catalyst Control Center at the top of the popup menu. I'm sure this is somehow the unintended consequence of the manual Windows install for the driver itself, rather than getting it done by the Catalyst Install Manager (which had failed to install the driver although it did install CCC successfully).
So I then went to Control Panel -> uninstall programs -> AMD Catalyst Install Manager -> change, and did a selective uninstall of Catalyst Control Center. That completed successfully.
Then I ran SETUP.EXE from the expanded 12.6 installer folder, and did a custom install to select only Catalyst Control Center. This completed successfully.
And now, once again, I have CCC on the popup menu from the desktop. And using it to check software, sure enough I DO HAVE v12.6 drivers installed!!!
I have finally emerged victorious, with v12.6 now installed for my HD4850!! Not exactly the simple install I was hoping for, but at least I've emerged victorious.
Still have no idea why the official Catalyst installers of any version no longer succeed in installing a driver on my system, but at this point I DON'T CARE!!!