1. The lower (small) turntable is there to only transmit the torque to the vertical 13L beam (liftarm) ... correct? If so, why such complicated connection between such turntable and the 13M beam?
There was a reason, but it was misguided haha
2. Also, why are you using 2L flextubes on the upper turntable? Might be better to use part 75535 instead ...
If that part is available in LDD, I couldn't find it [edit- found it haha]
3. What do the bionicle parts do? Transmit torque/rpm to the top turntable?
Correct. Otherwise, the pitch control links take the force...
[edit] As long as the swash plate remains centered on the mast, which the scissors should help do, and we keep the rotor RPM to a safe level, I don't think vibration will be an issue. We're not actually trying to make it fly, are we?