50 Amp leads are fine, (I havent measured the draw but the leads work) there is no problem using the lead as described Rob, I did that for Dave and my self.
I would say that the diagram is a very simplified switched/unswitched fusebox set up, and would work well.
As the cable for the jump lead is a heavy 50amp job, the use of the earth is fine, The idea is to stop the stacking of many connections on the battery.
Another way to do the same thing, would be to use another Anderson plug and simply plug in the fusebox system. Unplug to use the jumper leads, then plug in again when finished jumping. (dont plug in to a running bike but. If you were the jumper, shgut down and plug in the accessories and power up again. If you were unfortunate enough to be the jumpee, then go for a ride till the battery recharges, shut down plugin and start again.