The 'damage' to the underside of the nacelles is consistent with first points of contact gear up at touchdown attitude.
The GPWS 'Too Low Gear' would have been impossible to ignore though.
I would have thought that a subsequent go-around would have resulted in a tail strike.
Post crash fires are not always the result of remaining fuel. In the tragic wreckage site, there's more than likely enough burnable debris to cause significant burning.
Double engine failure due fuel exhaustion is a possibility.