ad 2.) The metadata backup (I'm guessing you're referring to the metadata.opf in the books' directory) is the prerequisite step to recover the database. The backup is automatically done by Calibre. If you want to restore the database from the libraries' folder structure, you need to invoke the
calibredb restore_database command from prompt.
ad 3.) Well, you can use it as such. If you connect to a folder, Calibre uses the folder it's connected to as a type of reading device, so you can send books to the folder using the send to device- action. Note that you can, for backup purposes, achieve a similar or better effect if you save your books to disk- that should, along with the ebook format(s) you select to save, also save the cover.jpg and metadata.opf files. Also note that another viable backup strategy would be copying a (known good) library folder tree including the metadata.db to a backup location.