Hi David, thanks for the reply and I enjoy your Youtube videos very much!
I actually was surprised by this request as well! But 2 of the 3 times I've mixed for boom operators (who were not me- haha!) they asked to hear the lavs as well. Glad I'm not the only one who thinks it's an unusual request. I'm told it's quite common, but also may be a preference to the region in which you're working? Dunno.
As to your video, wow that looks awesome! So- as I understand it, the Sound Deviced MM1 is doing several things:
A ) it is feeding 48V to your microphone
B ) it has a headphone out for the boom operator to listen to the boom.
C ) it has a line in, which allows them to plug in that receiver you're talking to them on, AND it is mixing that input with the boom input.
D ) it has a Mic (or line) output plugging into a TX they are wearing to get back to you.
E ) (not MM-1 related but a clarification) this means the boom op is wearing a TX, an TX, the MM-1, and talking to you through their boom mic and not a lavaliere they wear.
Is all of that correct? If so, I REALLY WANT AN MM1!
Sam