Thanks Leonardo
I rather foolishly followed the Microsoft OneDrive Support recommendation offered to you by Kumar S. to reset my OneDrive.
I was lulled into complacency by the fact that we have an identical problem and by the MS statement "You will not lose any data by resetting or uninstalling OneDrive."
Technically, this may be close to true, as resetting OneDrive did not wipe my data stored in the cloud, but it did erase most of my locally stored data, which was what I'd been trying to avoid and was the reason for seeking support in the first place. I did have some locally stored files that may not have synched to the cloud due to file name irregularities.
I followed the instructions to Quit OneDrive and double-click: ResetOneDriveAppStandalone.command
Immediately the local files in Macintosh HD>users>[user name]>Library>Group Containers>XXXXXXXXXX.OneDriveStandaloneSuite>OneDrive - [account name].noindex>OneDrive - [account name] vanished, although the folder/file structure subsequently rebuilt itself in a slow staggered fashion (from the cloud?). However the locally stored data from this folder is gone and files now contained within are non-functional shells.
At the same time a new folder was created: Users>rodger>OneDrive-UHN (Archive). However, this folder only contains some (20%?) and not all of the files previously stored locally in the folder above. Why some files were selected for archiving and others were not seems random. As least I could not identify a logical basis. This further fragmentation of my data into yet another folder is really annoying.
I previously had 400Gb of data stored locally in Macintosh HD>users>[user name]>Library>Group Containers>XXXXXXXXXX.OneDriveStandaloneSuite>OneDrive - [account name].noindex>OneDrive - [account name] (vs 600-700Gb in cloud), and now only have 70Gb in the Archive folder.
Fortunately I have a back up of my OneDrive from home and work iMacs (I need both, as I don't synch all folders to either computer).
At this point I'm going to set the new OneDrive (post Monterey) folder (or at least most of the subfolders), stored under "Locations" on the sidebar, to "always keep on this device" and have OneDrive download my cloud data to generate a local version of the files. This will mean downloading the 400Gb again, which may take quite some time. At least now I don't have to worry about having 2 local copies of these files on my HDD now, as the OneDrive restore deleted the previous copy (or 80% of it). I definitely need a local version of the files to keep my work efficient. I also need a local copy of the files in order to back-up these files up to an external HDD. I'll likely be missing a few files that hadn't synched from the previous local folder to the Cloud, and will have to hunt through my OneDrive back-ups for these.
I'll probably leave the Archive files lying around for a bit, in case there is a reason that this folder was created. But as many files will be duplicates of files that are downloaded into the new OneDrive folder, and as 80% of the files are missing from this Archive, and as it appears to be a non synching folder, I expect that at some point I'll need to delete it. It is using 70Gb of space.
NB. I thought about trying to merge my old and new OneDrive folders, to save me downloading so much data and to restore the local files that hadn't previously synched to the Cloud back to their proper place in my directory, but the behaviour of OneDrive is not transparent enough for me to know if this would work or create more problems - I might end up replacing some good working files stored in the Cloud with nonfunctioning shells files that OneDrive previously created and which appear in my back-up from OneDrive - [account name].noindex>OneDrive - [account name].
So for me the bottom line was:
Upgrading from BigSur to Monterey unexpectedly resulted in me having to restore my OneDrive folder from the cloud, and potentially loose local data that had not synched to the cloud. Fortunately, I likely do have back-ups of the non-synched local data on an external HDD; but these files (which were previously identified as having synch issues due to file names containing spaces or irregular characters) represent hundreds of files, distributed over hundreds of folders, and will now no longer be present in my active OneDrive account. As these files are scattered throughout my back-up data, its hard for to isolate them and just retain them, without retaining a full back-up of all the data. So my data is now fragmented across a new ONeDrive folder and a back-up, and in order to avoid data loss my data is duplicated. Not a good result from Monterey update / ONeDrive teams. Not sure who to blame.