Good questions....

I'm really not sure as to how the percentage of engine failures apply to Mosqito and Lancaster. The main problem with the Mossie was that single-engined takeoffs or - especially - landings were not for the novice. Takeoff and landing speeds were relatively high and any malfunction in one engine ( eg sudden mag-drop etc ) could lead to a sudden 'swing' due to high torque of the remaining engine and the light weight of the airframe. A relatively high number of crews did come to grief in this manner. Only skilled and competent pilots could handle a one-engined Mosquito landing ( although some, like the legendary nightfighter ace Branse Burbridge, did so on several occasions ).

The PFF Navigator was of course in charge of navigation, and also any adjustments to the 1154/55 radio which sat to the left rear of him. He also kept an eye on engine gauges, fired any Verey colours where necessary, and dropped the bomb (s) !

Apart from that, he enjoyed the night view, watched the props going round and round, and let the pilot get on with the hard work ( or so Reg Davey at the Mossie Museum, who was one, would have you believe.... )

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"Stand by to pull me out of the seat if I get hit" - Guy Gibson