Sorry, Mr Hoffman, for not getting back to you, but I have moved house and decided not to rebuild the MacOS while that was going on; fortunately (until now) there seemed to be no fallout from my scam attack.
But now. I have a weird problem - in Apple Mail, a seemingly careless keypress has removed the "All Mail" folder and I see no way to get it back. If there is a direct way to do that, please let me know!
So I decided to restore from a TM backup.
Following the support page instructions to restore the entire Macintosh HD from a TM backup failed, because modifying any of the folders at the HD level "are required by the OS and can't be modified".
So I decided to follow the MacOS User Guide "Recover … from a TM Backup" method
And in Step 2 "Reinstall Monterey" in the Recovery window, I get back to the fail because "the Macintosh HD cannot be downgraded".
It looks as if I will have to go back to your advice - reinstall MacOS but Ventura this time - and hope there are no more surprises!