This would actually make sense of it all. After I tested this yesterday I had problems sending messages to that one specific person for the rest of the day. It would fail, so I would resend it immediately and then it would work on the second time around. It happened with almost every mesage after the initial picture that converted the thread to MMS. When I switched off of Wifi, everything sent fine. After I deleted that thread and started a new one with that same person messages sent over wifi just fine again.
Maybe the phone is trying to do the same thing and after the thread is converted to an MMS message its reconnecting to 3/4g, but because of the issues we are having with the radios switching between data its causing the messages not to send?
Very weird issue I had never seen it before in any other phone.