Thanks, Mark.
Very useful - although that's the last lifeboat sunk - at least we know about the holes before jumping in it!
And yes, you are right - clean install seems only way out. So glad now I did it weeks ago, having wasted many hours of failed attempts to reinstall etc.
Clean install a major pain - but less stress if you think it through thoroughly ahead of time. Save copies of all your User folder, any application folders direct in C: (older software often used to use such for data and/or settings etc.), & your Desktop. Make sure you have license data for all your software, and so on before you Go.
Critically your W10 license number of course. Old W7 ones remain valid, at least on same PC (as opposed to HDD). Although at my previous generation change, did get help from MS, who agreed to transfer license from previous 2009 XP/W7/W10 PC to new one 2020 bought with test version of W10 installed, given that I was not going to use the old one post completion of the new.
And, as I said above, safest and most rewarding to install a fresh drive, clean as a whistle, no old junk files, deleted data etc. fragmented all over the place and so on. probably faster than your old one, and all your C: is available for fragments you may need,via either internal on a desktop or a USB carrier for a laptop drive.
NB Outlook a likely major pain - use Outlook itself to copy all your mail data, folders etc to a new Personal Folder which will have its own .pst file you can save and feed back in. Outlook is much pickier than it used to be about file locations for accounts etc.. Make a separate record of any Rules used in Outlook.
Good luck all.