Q1: If I restore a mailbox to a backup version, say from one year ago, what would happen with the current e-mail messages in that mailbox? A1: If you restore a copy of a mailbox from a year ago to its original location then the items on the old snapshot will be added to the current mailbox.
Q2: Would cloudbacko replace the contents, taking the mailbox and all the messages to the exact state it was one year ago, deleting the current e-mail messages received after? A2: No. It will add the old data to the current mailbox
Q3: Or will it just put back every e-mail message it has on that backup version, ending up with a mailbox that would contain every e-mail it has now, plus every e-mail it had then? A3: Yes
Q4: What happens with e-mail messages that were in the backup version, but still exist in the current mailbox, and what about messages that have been moved, subfolders, etc.? A4: Duplicate contents will be overwritten while items no longer in the mailbox but is in the old snapshot will be added back to the mailbox.
Q5: Does the restore behaviour changes if I choose to restore just an Inbox subfolder or a specific list of e-mails, for example? A5: Same behavior