Apologies for the delay in replying guys! i'll try and answer as much as i can...
I paid Â£350 for it from Halfords, but i did not get them to install it!
1. Yes it is, being parked or driving doesn't effect the unit.
2. I downloaded a video podcast to test, and no video was played back, just the audio was played.
3. No need to bypass anything.
I upgraded from an AppRadio 3 to the AppRadio 4 and to be honest, the size difference doesn't really make much different over the functionality you gain.
When i installed it, i didn't 'feel' it was a lot smaller, the screen is clear and bright.
I've been using the new unit for a week now and definitely impressed with it.
If anyone else has any questions please let me know..!