No reason to not think that the mini blades start in the center closer to the main blade. Besides it's sci-fi/fantasy. Why not just spend your time arguing how the force doesn't make sense while you are at it?
You are right. It can't get away with dumb. Thing is nitpicking Kylo's light saber design seems far more dumb then any failings in the weapon you have pointed out so far. Like I said earlier, you don't even know how it's supposed to work in the film.