There’s an additional proviso here too: They had help from the ground.
One thing that really needs to be answered is “was there an on-board fire in the cargo hold?”
That could explain quite a few things we’re seeing here.
An onboard fire large enough to disable aircraft systems will quickly weaken the airframe enough to cause a breakup, within minutes, not hours.
I think a cargo hold fire sufficient to kill the pilots would have destroyed the aircraft.
But (need help from knowledgeable FReepers here) there is an avionics bay under the cockpit which is accessed by a hatch behind the cockpit door. I don’t know if that bay has a fire suppression system. Suppose a slow fire started there, slow enough to kill the systems one by one. Suppose also that the fire generated enough CO to kill the pilots before it burned out. CO intoxication is very sneaky, it’s not like smoke inflation or toxic fumes.
With the cockpit door secured, and two dead pilots, could a 777 fly that long?