Well, it looks like a leader is slowly emerging in my quest for an Emacs based mail reader: Wanderlust
It handles MH- and Maildir-format mailboxes, UTF-8 in headers, has good facilities for reading and composing MIME messages, and I finally got it to work with Lotus Domino/Notes IMAP/SSL, amazingly.
Moreover, it looks like I'm probably going to switch to using a Maildir-format mailbox, which will work well with my use of Unison: MH-format mailboxes' use of plain numbers for message IDs/filenames (which can change a lot) makes things a little more confusing when syncing mailboxes with Unison.
Moving away from MH-format mailboxes is a bit of a shame, since it means that I'll be losing the ability to use the MH command line programs for accessing my e-mail. I've used MH in the past, and have always liked the command line access it gives you. Ah well. I wish mdmh had advanced further.