It’s a valid point indeed, it could be done within the regulations and meet the safety requirements. The GTN is divided into three parts, the GPS receiver and display controller, which is the most critical part, the VHF COM and the VHF NAV. All have individual power circuits, own wiring, own protection. The same is true for the remote equipment.
Any new audio panel will switch pilot to the COM (normally COM 1) during an emergency. You could easily check this on your aircraft, by switching of the audio panel and / or intercom, or pull their circuitbreakers.
When enough panel space is available, I would not recommend remote operation. The remote equipment is equally or priced over the panelmount equipment. NCYankee has a valid point. It does waste a lot of your expensive screensize, which could better be used for primary navigation instead of showing buttons in my opinion. You can find out for yourself buy downloading the Garmin GTN trainer software for PC or I-Pad.
The GTN-750 is much more functional in my opinion than the GTN-650, as the screen size is better suited for those large buttons required for touchscreen operation.
Isn’t having a remotely controlled intercom and a remotely controlled transponder taking a bit of a risk on that one single GPS (or just its LCD, etc) packing up?
Garmin has made some backup provisions, but it is a valid point. I personally would rather not use up the real estate on the screen to provide the button functions. I think the GTN750 displays more data in that space if it isn’t used for the transponder and audio panel. As long as you have the space for both, I prefer separate units. If my backup was a KX165, I think I would be more interested in having the GTN provided a backup power source. Peak current is higher on the KX165 and nominal current is reasonable on the GTN750 (1.4 amp).
Isn’t having a remotely controlled intercom and a remotely controlled transponder taking a bit of a risk on that one single GPS (or just its LCD, etc) packing up?
But, seriously, what happens when you lose power. I would want the KX165A+KI204 to be switchable direct to the battery, and I believe there is a mode in the PM7000 intercom whereby it forms a pass-through when it has no power.
Any modern audio panel / intercom will switch you to COM 1 on failure / emergency, this is part of the certification specifications.
Ah, so you don’t need one of the remote transponders such as the GTX33? Didn’t know that…
You don’t need to have a transponder to use the GTN series. When you want to use the remote transponder feature you must have a Garmin transponder such as the GTX-330 or the remote GTX-33.
Both GTN 650 and GTN 750 are able to control a Garmin transponder.
Only the GTN 750 is able to control the GMA-35 remote audio panel.
About the third radio PMA7000b can handle 3 comm’s I believe
Ah, so you don’t need one of the remote transponders such as the GTX33? Didn’t know that…
It sounds like I would lose the intercom and lose the 2nd radio, so could move the transponder and the ADF to the centre stack, and put a Kenwood RNS310 satnav where the transponder+ADF currently live 
But, seriously, what happens when you lose power. I would want the KX165A+KI204 to be switchable direct to the battery, and I believe there is a mode in the PM7000 intercom whereby it forms a pass-through when it has no power.
I also really like the separate intercom and all the real physical knobs on it.
No need to move the transponder. A well configured GTN750 can drive the GTX330 if also adequately configured. Better to have one central place (GTN750) to enter all information coming from ATC.
I would just get rid of the 3rd radio and move the transponder across, for convenience.
The GMA-35 can be installed behind the GTN-750 screen and features 3x COM support. 
