"- Some way to read my backed up emails easily without re-importing them to Gmail - a mini mail server, a web-based UI, etc."

I'm using offlineimap, which syncs an IMAP server with the MailDir-Format. The MailDir-Format should be readable by any email program.

thanks for the support. You can backup multiple accounts in one data directory (Gmvault-db) with the option --multiple-db-owner. When you restore though all the emails from this Gmvault-db will go to one Gmail account. If you want to have the possibility to restore each account individually, then you have to use a separate gmvault-db dir for each account. In the roadmap for v2.0 I have some tasks to add a better support for managing multiple accounts. As you said all the accounts will be incrementally backed up automatically onced configured. v2.0 will provide a GUI and one feature should be to allow users to browse their emails. A clean way to export emails in standard format (MailDir and maybe mbox) is since this morning also in my roadmap.

If you want mbox you can try www.snapshottool.com, which runs in the cloud and provides a local download.

You can already do that by 7zipping the subdir db of the Gmvault-db dir. I think that 7zip can archive a directory tree. You could also tar it and 7zip it to have one dir containing everything.