I am starting to install my Tab into my Xterra too.
1. The reverse camera will work with Wifi but there is too much switching time is too long on startup. I seen someone testing it and it took almost a full minute. He was able to cut the time down by different ways of starting up the router and modem but not by much. The only way to cut it down is by remote starting the car every time you want to drive the car.
2. How are all calls received to the tablet? The only app I found is Tablet Talk but that is just used for notifications but still goes through the phone. Plus that only notifies you that you have a phone call and you are only given 2 options none of which to actually accept the call and talk.
I don't know how much power is drawn but the OBD2 Bluetooth adapter will be powered 24/7.
My question to you is, do you have steering wheel controls and if so, are you going to try to use them?