“In the world of ancient or medieval combat, lefties would be at a distinct disadvantage having to use stuff designed for right-handed warriors”
I would think that this point is obviously correct right up until the defensive formation breaks down (and it always does). Once one side ‘breaks’ that is where the killing really begins. And that may be where the left-hander gains an advantage. I don’t know this to be true, but I’m suggesting a reason that Left-Handedness managed to survive the tens of thousands of years of muscle-powered weaponry.
Again, there’s no such advantage. If there were, it would be easy to explain, like a three-armed warrior would have an advantage.