yeah dude. the ddin conversions are expensive as hell. especially if you're looking to maintain steering wheel controls. there are adapters left n' right you gotta buy after the kit and the unit.
but to answer your questions:
all your steering wheel controls are still functional.
the unit does not effect your current setup. the two are independent of each other.
yes.
I cant on mine and i bought it from the same company. To be honest i use the included remote control to change songs, using the unit itself is too difficult. You cant scroll through the list easily (actually i cant work out how to scroll at all other than pressing fast forward to play the next song)
Yes, your 6 stacker will still work.
Not for playing music
Im considering hooking my ipod up via the AV inputs. This would result in a black screen of course but i could then mount my ipod somewhere and use that to change music.
EDIT
Ok i just routed a composite to 3.5mm cable through into my cupholder and connected it to the inputs in the back of the system. Now when i press the AV button, my ipod is my music player! Then you can just close the systems lid and bobs your uncle! No steering wheel controls sure but an ipod is easier to fiddle with than the on screen players.
This will now be my preferred music playing method. $600 might seem expensive for what is effectively an AUX input haha, but the GPS program is actually really nice (iGo8) and playing a dvd might be handy for long trips.