Monitoring the microphone through the headphones is a very important feature that is missing in prime 2 and present in prime 4
I think given the fact it’s a physical button on the device it’s going to be struggle to get it implemented. But they should definitely look at adding ‘cue’ to the next version of the Prime 2.
I guess a workaround could be a small sub mixer connected to the Aux input if you really need that ability. It’s not ideal but it gets you somewhere near what you need.