I'm not sure I understand why calling the DM line doesn't resolve this issue very quickly. When I fly internationally, for example, and want to book the over the water portion in M, I have no need to book the domestic portion that way. So, I find the flights that I want, call the DM line, and ask them to break the fare. They make a call to their fare desk and get it all booked. Lowest fare on the domestic leg, M on the other. If they aren't doing this for you, call back and get someone else to help.