Thank you so much! I haven't tried yet, but I'm almost certain this will work now. I don't recall when exactly I left FirmwareFeatures and FirmwareFeaturesMask blank in PlatformNVRAM, but it must have been weeks ago. I think I must have misinterpreted advice by
@tsialex or someone else that the presence of those values might eventually degrade the NVRAM chip. As for BoardProduct in DataHub, I doubt that ever was in my particular setup, although I may be mistaken.
So, I've changed all these parameters and try them out later today. Thanks again!
EDIT: Yes!, that was it. Success at last! There was something odd, however. There were no indications whatsoever as to time left. No messages saying things like "Estimated time remaining: 12 minutes". The computer rebooted four or five times and then went straight to my 12.5 desktop. Excellent. Many thanks!