Re key commands: Doesn't it? I know that the Cues don't have keycommands for (most) everything, but I thought that our MC key commands were complete (maybe not for the speaker solos/mutes)

EuCon monitor support is pretty complete; it has full support for virtually all the features of the MC and it does show and set level. It may be that the EuCon devices you have used have limited MC support. Give it a try with the free Avid Control iPad/Android app.

It all depends on what you are used to... I'm used to and like to work with a monitoring section on my mixing desk or sitting on my table where I could change sources, speakers volume etc. and get a visual feedback like on every professional console. Key Commands are working fine when I want to control the Monitor Section, but they will never give feedback except in the MC window.
Regarding EuCon, unless you own a S6 the monitoring section is really not usable, since you have to go to a subpage on the Avid app. You can only have one App connected with EuCon, so you would have to keep that page open if you want to achieve the workflow I mentioned above. (and there are so many better things in the app to waste it for the Monitoring section). EuCon would also be impossible to customize, since Avid would never allow any 3rd party EuCon controller...

RME is using simple Midi learn for their TotalMix along with Key Commands.

I think having MCU already implemented it would be quite easy to make a different mapping, like Master Fader = Volume, Track Select = Source, etc. as long as they have LEDs on the MCU that can be driven. The MCU protocol is available to everyone and very simple to program with boards like the Teensy. Just add a couple of buttons and LEDs and an encoder and you have a nice Monitor Controller.

The monitoring section was a big selling point when I bought the LIO since my monitor controller is only capable of 5.1 (but I'm still using it because of the lacking remote control features)