>>The hijacker pilot can fly the plane manually, but not by autopilot and not using GPS. He would only have a backup compass to steer by. Everything on a screen can be darkened by the pilot/engineer in the service bay. One plug at a time, the hijacker’s job becomes more and more difficult.<<
I don’t think it is possible to fly a 777 in the dark of night with a dead panel. Having a magnetic backup compass for heading guidance is fine but maintaining a level fight without a speed indicator is impossible. Just one degree off will result in either overspeeding or a stall.
Two comments on your post.
1. Pitch plus power equals performance. You set the power, maintain a pitch attitude in reference to the horizon and you keep flying. Maybe not at a perfect altitude or speed, but you stay in the air.
2. The old parable of the scorpion crossing the river on the back of a frog. If “the good guys” disable all the nav and flight info to the flight deck, then they defeat the hijcacker at their own peril. They might prevent a 9-11 flight into an innocent target, but they just sealed their own fate and those of the passengers.
The 777 has backup indicators. They are in the center panel.