thanks all....
so I opened the guest user account, added the desktop in the finder sidebar preferences, logged out and then reopeneand had no issues with it disappearing.
I'd already figured out where the files got moved to, and have them all restored to my user desktop folder.
But that's the folder that now won't stay in sidebar: in fact it does this weird thing: in the finder;preferences;sidebar, when I click desktop, it unselects itself immediately. I click again, and it will stay selected, and appear in the side bar until the next time I log off, when its gone again.
as noted desktop is fine, files are all restored after getting moved off, i've disabled all icloud synchs of documents and desktop....