How to export all emails from a mailbox in Apple Mail
I couldn't get my export working even with a mailbox rebuild.  However, my goal was simply to archive my old mail, and not necessarily to do so in the mbox format, so copying the raw mailbox with 'mv' was adequate for my purposes.

After some trial and error, I found the path was ~/Library/Mail/V3/IMAP-&lt;account-name&gt;/.  Sharing it here in case anyone anyone else is in the same boat.