Restoring a single mailbox is actually pretty simple if you’re using Virtualmin, provided you know the command syntax.
The issue is finding the command syntax as it’s not properly documented in the manual. You have to google.
The solution is here:
virtualmin restore-domain --source /path/to/backup.tar.gz --domain whatever.com --feature mail --option mail mailuser [email protected]
The part that’s not in the manual
mailuser, was found on their forums.