I think a fire on board can be ruled out, there is no way it would have continued to fly for such a long time with a Fire on Board, although perhaps there was a localised fire that produced toxic fumes and then petered out or was extinguished. Also control and power circuits would go in a big fire, and Al melts real easy as anyone who has tried to weld it knows, so the fuselage would be compromised.
The range is consistent with the fuel load, what I want to know is are/were the relevant tanks auto-selected as required? Back in my piloting days, I had to manually select wing and center tanks to keep the aircraft balanced (single engine)
But in aircraft fires, its the fumes that kill long before the heat.