1. Surely you've got that backwards? The magnets are intended to be used at saturation in pick-up mode. Here we will run them well below saturation, hopefully somewhere on the linear-ish part of the B-I curve. I'm only using the saturated value of 1.6T to find the max possible retardation.

2. Ok I get you now. At the nominal design speed of 60rpm I guess there will be 'vibrations' at 90Hz from the planet/sun wheel interface and 60Hz from the ring/planet interface. I expect these will be small - I guess they'll depend on the quality of the tooth shape and the fit of the gears. We won't know til we try! :)