A few thing bother me about the 'error'
Can you create a
New Library On the desktop and Not have it be a Single folder?
The Desktop would have to be 'empty' for Calibre to make a New library. The user would then see all the Authors folders and Metadata.db as books were added.
The other case, I see is the user copied the contents of an existing Library and dropped it onto the desktop

. The Calibre would not stop them from switching to a 'existing' library (or removing it).