How do I safely restore files to a shared folder after they were replaced with symlinks created by iCloud's "Optimize Storage" feature?
Yes, it's a pickle to have two competing cloud drive sync's running on the same files.  You need to ensure that iCloud, Google, Dropbox, Box, etc. are not using the same locations.
iCloud sync's the Desktop and Documents folders so make sure none of the other competitor cloud drive sync folders are not located on the Mac desktop nor documents folders.
I would have the problematic user disable the Google sync and download the documents from iCloud then move them to a new folder outside the Desktop or Documents folders.  Then ensure that the folder that Google uses to sync is also not in Desktop or Documents. Then enable the Google drive sync new location.  Move the files into Google drive sync folder and let them sync back to Google.  That should fix the problem once the other users sync them back down from Google.
If more than one user has turned on the iCloud Desktop &amp; Documents sync feature they need to do the same thing. Except now someone has to figure out what's new versus old in regards to the files that have moved into individual iClouds over a period of time. So if that is the case, ouch... That's painful if it's a lot of files.
Decide on a single cloud solution and stick to it. Do not mix your cloud solutions.  This sounds like an accident. So hopefully it was just one user who did this.
Check the Google drive that's on the Google Cloud see if the files were all replaced with .cloud links. If so then the master copy should be on that one users personal iCloud.
Once you've resolved the Google mess.  Then the user can either continue to use iCloud or they can turn off the Optimize feature, download all their files (if they have enough local storage) and then they can turn off the iCloud Desktop &amp; Documents feature if they really don't want to use it.  Otherwise they need to keep other cloud systems outside the Desktop and Document folders.