Don't forget Churchill's stated reason for picking Normandy. The Germans assumed the shortest distance is the shortest distance, but overlooked the fact that there was no way all the shipping could come out of the single port of Dover. Although geographically Calais was closer, the best ports for departing shipping were in southern England. Therefore the difference in distance was pretty much a wash.

As for what would happen if the Allies landed in Calais, I think they still could have eked it out. The Allied air forces would have had a field day on German reinforcements in the more open terrain, and the shorter distance to airfields in England would have helped. The first few days might have been bloodier, but in the long run the breakout would have occurred sooner, with the potential to bag large German units in southern France and a more realistic chance for something like Market Garden to work.

__________________
Sole founder of Encyc.org, now covering World War II.