I always like seeing swashplate ideas - I appreciate all the documentation you’ve put together, too! Fun to see my helicopter referenced (which borrows heavily from steph77’s).
This puzzled me too for a while, but I think a section of the rotor axle in these models has been shaved down slightly, to allow the ball gear to slide. Obviously not “kosher”, but the solution is certainly compact.
I’ve seen many attempted designs that started at this point, but couldn’t find a compact solution to constrain the motion. I’m excited to see what you come up with! Of course for my Land Rover C-model helicopter no turntable was available, but my clunky solution does lock the upper swashplate to the main axle, at least, while allowing up-down movement. No such luck locking it to the lower swashplate, which may also be the case with your preliminary design in the video. Still, how often does a helicopter fly upside down?