I Looked deep into the innards of calibre.db
Authors_Books_Link has the start
of what you need.
1st Needed is the List of all Author (id#s) that have a count of only 1 book
Next you need to get the Book (Id#) for those Authors, but does not have another Author record with the same book (id#)
My SQL foo was not up to this (getting the list )
once that list is complete, you need to export the list so it can be used by a script to cause the Books to be moved. That is another problem.
I don't see a CLI that has Copy to Library (or an import of a List to 'Mark' books for you to the use Copy to Library
As you can see... Not simple