SERIOUSLY
DO NOT try and back door this. You have a 99.9% chance of making a disaster.
Create Switch: Use Existing. Easy to use, Just copy the path of where you found the Metadata.db (spin and repeat for each Library)
Before that, Note that the folder containing metadata.db IS the
Library name.
Make those name unique to avoid any confusion. <<<
FWIW Calibre Library is NOT one of my active Libraries. If Calibre ever starts with that name, I know the configuration
folder has damage (that is where the list of KNOWN and Last_Used Libraries is stored. That does not indicate the Library is damaged