iMac Documents

It's not there and won't let me add it...says "The name “Documents” is already taken. Please choose a different name.", but I don't see it.

See my post.

The folder is still actually there, but Finder is hiding it from you since you set up Desktop & Documents in iCloud Drive.

Did you turn on (or fail to disable it) Desktop & Documents in iCloud Drive when you ran the Startup Assistant?

If it is enabled, your Desktop and Documents folder will show up under iCloud Drive.

/Users/<user name>/Documents

Did you read Barney's last post?