Putting one's keyfile in the cloud just seems to me to be asking for it. You're essentially trusting a 3rd party with the keys to your kingdom.
* The database in encrypted with your master password.
* You can optionally also encrypt it with static "Key File" that are on all your devices but not in Dropbox.
B) You choose which 3rd Party to trust. There are many options with different security/trust/threat models.
(Example: lately I've been using an encrypted share in Resilio Sync where the "cloud" option for me is a dumb VPS that can share the folder torrent but does not have decryption keys into the contents.)