Since this writing, I am no longer using the iOS or desktop versions of Mailbox. There was no movement or communication from Mailbox or Dropbox, and I took this as a sign that the software has been completely abandoned.

I switched to Airmail 2 for desktop, and reverted to Mail app on iOS. Airmail has made great strides since the last time I used it, and they claim to be developing an iOS version as well.