Regarding the rotor brake, it is both common and allowed to have the brake on efore starting one or both engines, then release the brake after the engines stabilize at idle. In high winds, it is preferred to start against the brake, to minimize dwell time at low Nr and low acceleration.
It is also allowable to shut down while one engine is at idle and the other shut down, thus keeping one DC gen on line and assuring a fast start. This procedure does wear the brake a bit faster than a shutdown with no engines on line, but I don't think it is a major difference,
The flight manual normal procedures describe these runups and shutdowns in detail (bell4can, I wonder how you got to sit in front and don't know this!)
Regarding MGB design, I believe the Flight Safety package shows a sketch of the transmission gear train (with their rpm's) I will hunt down my copy (my garage has a dozen boxes of prime stuff waiting archeologists to explore it!