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phossler - AFAIK the metadata.opf file in a book folder only serves one purpose - as a backup of the what's in the metadata.db database. My understanding is that it's only read by the Library Maintenance->Restore database function.
Any similarities between its content and similarly named components inside an ePub are coincidental. The book editor can be used independently without the presence of any calibre libraries.
Mentally, I rename the metadata.opf files in the book folders to calibre.metabak, and turn on the Hidden and Read only attributes
BR