No to all 3.
You can have as many LOCAL drive Libraries as you want
(but there are usually better ways to get the results).
They can be anywhere on your system EXCEPT you can NOT put anything else inside the Library folder.
(folk will say, THEY put it in the cloud...But it works till it fails someday.)
Virtual Libraries are VIEWS (all in a single Metadata.db) of the base Library, not individual.
Once added to Calibre, just archive the original off site. The copy method used allows for burps along the process, and a safe retry.
Gimme a break. save drive space? Multi Terabyte (spinning) drives are under $100
(A 12T drive sells for less than my first Seagate 40M HD cost me , no adjustment for inflation). Clone software comes with most retail drives, I found Clonezilla did the job on a dual boot system, when the 2 common ones failed to do all partitions.