couple things I can put in here, can't answer it all though
For your touch screen mouse control concern, it'd work the same way if you used two fingers on one touch screen... it'll jump around.. if two people clicked something at the same time, one of them will work, depends on who got to it first
Powering would be best with a DC-DC converter, such as the one I use, M2-ATX.. it'll give you shutdown controllers and such too
HD vibrations I'm looking into right now too, because I'm planning on building my carputer into my sub box. One of the ideas that has been tossed to me is mounting it onto something that is already as solid as possible, then using vibration absorbing foam, like Dynamat
Temp, I live in Canada, same concerns here. I've been told it's not too much of a concern, but I'm thinking of integrating a circuit into my car so whenever I plug my block heater in, it also powers a small USB cup warmer or something, in the box, so it keeps a certain level of heat in the immediate area.