single cobblestone block would be nice

Actually, it's not just in the corners where you can overlap the cobblestone walls. You can overlap them anywhere; if you just want a three-square long wall, you can overlap two straight cobblestone walls.

Cobblestone walls are awesome; I wish EA would change all the walls, fences, and hedges to be overlappable. (Even better would be to make them like roads, extendable ad infinitum, but baby steps).