I don’t know if you consider this? If your using multiple tethers to form a shaft then you might want to take a leaf out of rope/ cable makers hand book. Is there a way you can take all those torsion forces and consolidate it into a solid shaft? I know when twisting rope it tends to like to bind up on itself. This would all so make for a stronger tether. I see no easy of avoiding torsion forces so there must be a way of getting something better out of the situation? Medieval rope-making in Visby market - YouTube
It reminds me of torsion spring. It just about find a way to absorb those forces to achieve the desired results. In effect have it connected to a bike wheel. With it splayed at the bottom, tort at the kites attachments.