Not sure I agree with you, BPF. There are several training benefits of going right down to actual Vmc which I can think of. Safety is, of course, paramount, and your story of the snap roll is sobering, but - my personal opinion, I have no problem if you don't agree - there is no safety issue demonstrating Vmc when I have control of the elevators, ailerons and throttles.
Depends on the aircraft, doesn't it? Vmca and Vs (in the practical rather than certification sense) are determined by very different features of the aerofoil and controls. On some aircraft, it's easy to reach Vs before Vmca.
In part because it is quite low-powered, the Twin Comanche on a single-engine (and at any altitude at which you would contemplate doing the exercise) retains sufficient rudder authority to maintain directional control right down to the stall. I think there were some nasty accidents in the 60s and 70s involving Vmca "demonstrations" in training situations that resulted stall-spin upsets, and it was as a result that the POH Vmca of the Twin Comanche was raised to the modern 78 KCAS figure, to give the instructors and examiners an excuse to terminate the demonstration after slowing to that speed.