Just my thoughts,
Wouldn't a partially deployed or recycling thrust reverser (on one side only) explain the extra 500 meters of runway used before lifting off? If that is a plausible scenario, why was there no reverser light or warning indication on the flight deck? Apparently the crew had no notion of anything wrong with the aircraft during the take-off roll.
And could the "bang" heard by some survivors have been the reverser extending to fully deployed position just after lift-off?
Green-dot