As for the controls moving to the neutral point in the event of a double hydraulics failure, we have done these failures in sim, and there is never any movement through any of the controls - they simply freeze in place.
As always, the sim may or may not model that accurately.
That neutral point of the elastomerics is without any aerodynamic forces, until someone experiences this I don't think we can be sure of what would happen in a full hydraulic failure. I do think that frozen controls is the most likely outcome, but I would expect some feedback.
If the interlock fails, just move the switch that you last moved. Your finger should still be on it.
Some of the AFCUs accelerate quickly to idle from the “about” 35%N1 position.
If the engine is accelerating too quickly, that should be adjusted.
When I see a range such as 77-85%N2/Nr I usually target the middle of the range. Assuming that there is a reason for the maximum, as there is for the minimum, I choose to avoid both extremes.
Following what the flight manual says, and not strangers on the internet, is a good idea.
Matthew.