OK some great news for those that care about the rear screens...
Geoff's recommendation for a wireless mouse was excellent. I drove down the hill to Staples and picked up this wireless USB mouse...
This allowed me to control the EC environment exclusively with a mouse if I desired. No setup. Plug in USB dongle to USB 1 or USB 2, pull battery protector tab on mouse. Use mouse. Mouse arrow shows up on the rear screens.
I started with the rear screens in AV and it controlled everything in the EC environment. The front unit changed as well when it is in AUX mode. So if I changed the app in the back it changed the front to the same. Audio and video. Doesn't seem to be possible to use 2 different android sources split front and rear at the same time. I think that's reasonable.
But then I changed the source on the front screen to SAT and it switched to the OEM system, and I left the rear in AV. I got SAT radio on the front with audio from the OEM speakers and the rear screens remained on the EC unit. I could then use the mouse to make changes on the EC unit which no longer affected the front...
CarPlay in rear with SAT/NAV split screen in front...
Android home screen on rear with SAT/NAV split screen up front...
Netflix on rear with SAT/NAV split screen up front...
The one thing I could not figure out how to do from the rear using the mouse was escape CarPlay to get back to the android home screen. On the head unit up front you push the home button while in the EC mode. Not sure how with the OEM system running up front. Getting back to the android screen from Netflix is easy as there is an available back button.
I think this is pretty sweet. I don't know that I would use it a lot but some others might find it very useful. Just realize this is me sitting in my driveway putting this thing through it's paces to try to test everything out, not real world trips. Your mileage may vary.
I have not heard back from EC on the ghosting. We would almost exclusively only use one screen at a time but we would certainly like the option for the kiddo to sit in either seat on a trip. I will update with any suggestions they have.
Also of importance is that I have noticed a loud pop (volume of pop determined by volume setting) when first engaging audio for CarPlay. If the volume is high it's a loud pop. Volume off no pop. Swapping to any other sources or back to CarPlay after initial startup it does not happen. Starting in any other android app it does not happen until I select CarPlay. I will double check Netflix to be sure. No pop with Media Monkey. Shut the car off and restart... pop when audio activation in CarPlay.
This bothers me a lot. I did not notice this initially because of 1 of 2 things in my estimation...
1. It didn't pop previously and after the NAV voice update it created this problem (maybe unlikely but I feel like I would've noticed this right out of the gate). It's concerning.
2. I never had the volume up enough to hear the pop when I first activated audio in CarPlay. I find this hard to believe but I'm not betting any paychecks on it.
I hate to seem like such a pest to the EC guys but I am going to send an email asking about it. This actually would make me consider pulling it out.