To fix the rest, I went into ~/Library -> Mail -> Mailboxes, and control clicked the imported mailbox and selected "Show Package Contents." From there, I opened the "mbox" file in BBEdit and removed all of the '>' characters from the beginnings of "From:" lines. After this, all of the messages imported without a hitch.
So if you're having trouble importing mailboxes, you might want to try this.
[Editor's note: I have no idea if anyone will find this useful or not, but I figure if you're stuck trying to import a mailbox, having this knowledge around can't hurt...]

