1.) The controls need a piece of hardware to control it. Something like the Fusion Brain. You need something to go from the virtual world to the physical world, which an USB I/O Board would do.
2.) Well... ish. There have been a few people successful with integrating their OEM screens. Things you will need to overcome is the touch overlay not being USB and the video input not being VGA usually. There was an instance where an OEM screen was a 640x480 and he got VGA into it but with lots of research and time. Usually they are composite monitors (like a dvd screen). That means with a computer it will be blurry. Useable but blurry/fuzzy. For the touchscreen, if your OEM TS overlay is a 4 wire, then you can plug that into a USB TS controller (~$50) and then the presses will work like a mouse with your PC.
The particular case I keep talking about the guy actually used a bunch of relays to share the screen with the stock system and the CarPC. I suggest you try to do that first IF you are intent on keeping your OEM screen and most stock look. That way you can "switch" between the OEM stuff (including the hvac controls) and the CarPC. You need to have a switch that trips relays for both the touchscreen and the video input, or else your presses wouldnt coorespond to what was on the screen at the time.