An override from the ground means an override can be transmitted from the ground, or any place else a terrorist happens to hold and override console.

Right. The pilot would only have this option if he or co-pilot thought were going to be possibly disabled by terrorist. With proper access code the pilot should be able to override any computer or ground control. The access code could be unique to each pilot and required before starting any plane. And an additional check could be that the flight tower would need to validate that code before the plane is first used and once validated only that pilot could cancel the authority. Therefore a pilot could cancel this authority mid-flight- requiring that pilot (or another pilot) get his code validated before a pilot can have "primary command" returned.