As for your first post, yes, I believe all of that would be compatable. As far as I can tell, everything is using standard A/V jacks. But, to be honest, you are spending an incredible amount of money. The DVD navigation includes a touchscreen, and so does the DVD player, you dont need both. The DVD navigation already plays DVD's. Why do you need another DVD player? Furthermore, for the price of the DVD navigation, you could easily build computer, buy a touch screen. Buy a TFT LCD, and a headrest mount, and have quite a bit of pocket change left over.
DigitalWW has everything you need for a headrest mount, and considerably cheaper than the Clarion screen. Hell, a USB GPS is very inexpensive when compared to that navigation system.