"The 2 FADECS for the engines are indeed independent but how about the autothrottle itself ? Is that perhaps a possible single point of failure ? Just asking."
The A/T really isn't an issue here. All the 777 Autothrottle does is physically move the thrust levers in the cockpit using little electric motors. The thrust levers then indepedently send electric (thrust lever angle position) signals directly to the EEC's (or "FADEC's" as you call them). As stated before, the pilots manually advanced the throttles with the same effect (none).
Hope this makes sense
