Why does the DC-9-80/MD-80 have this cusp in its fuselage?
It's not a stress concentrator; it's just the opposite.  What you're missing is that the floor itself at the pinched part forms a tension bridge that allows a more or less 'ovalized' circle while still maintaining tension loading on the skins and frames as if it was a pure circle.  

If I had a rubber balloon filled with air and was able to run a string internally from one side to the other, attached to the walls of the balloon, and then drew the string in to pinch the sides of the balloon into a figure eightish profile, I'd have the same thing.  All the loads on the skins are still in tension, as well as the bridging floor beams (the string).