The basic architecture and torsional stiffness, and the cylinder heads. Technology advances taught GM to roll the valves back to 15 or so degrees. The adaptability of the ECU's was a huge help as well.
That's exactly why. Same when an aftermarket tune is uploaded.
I don't think resale will be affected much. If at all. VW will weather the storm and move on. Actual cost of it is a matter of a reflash will be minimal. The governmental fines are be a hard pull after much litigation and I would bet there will be plenty.