You can forget about being able to do new stuff via calibre companion, because calibre companion access to Dropbox for new devices is broken and new developer has no intention of fixing it.
But have you considered setting up a folder in Dropbox which is configured as an add books folder for calibre, so anything you upload to that folder, from any device with Dropbox access, will be synced by Dropbox to the local folders of the device where calibre is running, and thus auto added to calibre
It should be possible to tell calibre that C:\myuseraccount\dropbox\intake. Or whatever is an auto add folder, and any Dropbox app or logged in browser can write to it
|