Great to see an analytic approach to multi kite rotation.
My gung ho experimentation led to this test which likely confirms your ideas on the benefit of lateral tethers coming from the inner tip of kite blades
There’s 2 rotors in a TRPT stack here
The top rotor is intact
The lower rotor is disintegrating, The main ring snapping behind the trailing edge of the fuselage.
The main difference between the 2 rotors was where the radial centre lines attached around the ring. On the top rotor 3 of the lines attached to the inner tips of the 3 rotors, the other 3 lines attached to the subsequent midpoints of the ring.
On the lower failing rotor the lateral lines all go to the ring (and ahead of the blade fuselage) … All the centripetal force on the blade fuselage was torquing the blades forward around the lateral line connection, resulting in too much stress on the ring behind the fuselage.