It's also possible that if you keep calling and get the "wrong" agent, that agent might decide your existing itinerary is invalid and cancels it.

Getting an MPM exemption from UA on an award is more likely if the majority of the requested routing is on UA-metal. I had that happen on trying to get DEN-xyz-SCL. UA adamantly refused to put me on AC as a single-connect via YYZ even though they saw the AC availability. It was 23 miles over the MPM. I was able to talk a supervisor into finding me something to SCL, but they insisted on a DEN-IAD-EZE(UA)-SCL(AC)/SCL-GRU(LX)-IAD-DEN(UA). So the bulk of the flying and all of the intercontinental was on UA. Even though it was about 1500 miles over the MPM!

Good luck but I wouldn't count on getting what you want, and what you have may be at risk if you keep trying.