Any question answered...

I don't think I have a socket big enough for the Rotor/P2M spider. How can I get close enough to their required 40-50Nm without resorting to the "until it breaks then back it off quarter turn" method?

Buy a big enough socket.

Their own instructions are showing the spider lockring tool in a vice and then tightening via the crankarm. How do they torque it right?
power2max.de/downloads/power­2max_type-s_manual_web.pdf

They can't. I suppose you could put the tool in a vice with the chainrings vertically and then use a calibrated weight hanging from the end of the crank to get the torque just right. Seems like hard work though.

How do they torque it right?

Put some softjaws in the bench vice, clamp the crank arm with the lock-ring facing up, put the lock ring tool in place, connect a very large torque wrench, then torque to spec.

I usually do this stuff at an LBS...not because it's difficult, more because I don't have anything study enough to bolt a bench vice to that will then take 50nm without getting destroyed.

I think @dammit has used those black n decker workmate benches to do this stuff in the past though.