Regarding (a), "if I applied no external forces or torques to my system, and now the thing is spinning...", of the original post; sometime before T=1s, the experimenter compressed each of the springs with a required force, thus clearly, an external vector force (torque about the fulcrum) was...
In the TWT, a stimulus rotating EM field modulates a DC electron beam naturally accompanied by its orthogonal magnetic field, thus the modulated electron beam itself launches a generated EM field. The EM field generated by the modulated electron beam is collected by a wave guide to the device...
Assuming the base of the enclosing vessel is fixed, both hot and cool gas molecules under an external gravitational field have a fixed floor beyond which they cannot travel. Rather than coming down, more appropriately, molecules of a cooler gas just do not reach up (against the force of gravity)...
Isn't the high energy laser-electron interaction described in the following article and citation, the very same principle used in the classical TWT amplifier?
https://phys.org/news/2018-02-intense-laser-evidence-electrons.html
https://journals.aps.org/prx/pdf/10.1103/PhysRevX.8.011020