Homebrew mutt(1) file permission weirdness
Ok, I still don't know what's going on with the .mbox thing.  I suspect that's a mutt/mail config issue of some sort.

BUT... I have a solution for the inability to write /var/mail/username.

http://hints.macworld.com/article.php?story=20100609140550467

dseditgroup -o edit -u john -p -a john -t user mail
dseditgroup -o edit -u john -p -a conrad -t user mail


(john is the admin user)

I added both users in question to the mail group and now both users can use mutt (no command-line arguments) to read their mail spool.

(As an expedient hack to get rid of the visual flag/error message for .mbox, I did:

touch ~/.mbox
chmod go-rwx ~/.mbox


)