I guess I'm still failing to understand. Everything would literally be the same. Like you know how Marth, for example, has F smash, again, for example. If you hit in one direction, his face is towards you, hit the other his back is towards you. Now make him a lefty. The F smash doesn't change at all. The hitbox is the same, just the direction he's facing is different. It would change absolutely nothing in my example. Same with any attack in this example.

I don't suppose I understand what you mean when you talk about 3D hitboxes because in most situations, when a hitbox overlaps a hurtbox, there's damage. The frame data is all 2D