Aircraft collapse
From a structural standpoint, it appears to me that the aircraft collapses (nosewheel and fuselage break over wing box) occurred at a standstill, and were the result of hot fire gasses rising to the ceiling of the upper deck. This could occur from a fire anywhere within, and in my experience that fire would not have to have much fuel, just a little time--about ten minutes after taking hold.
Reasoning as follows: In a totally reinforce concrete house built as a fire training facility, a fire was set in an upstairs master bedroom with two shuttered windows and an interior door, all closed (heavy green wood). The fuel was three wooden 4x4 shipping pallets and a compact bale of hay, a little newspaper. The room had a special fire lining under test, ventilated from behind through the flat roof. One vent was instrumented. In 5 minutes, 800 degrees at ceiling was exceeded and ceiling flashover occurred. In less than 10 elapsed minutes, vent temperature reached 1600 fahrenheit. A fire crew of 4 to 6 firemen entered the room from the hall at this time under the fire gasses burning in the top part of the room space, and extinguished the fuel with a fire line. One had the top-of-ear burn as a result.
I think that heat to this extent will either burn or melt the top crown of the fuselage tube. Lesser heat at the side will considerably weaken the structural strength in tension of the tube. This tube is a longitudinal beam, and the burned or melted top quarter is its tension side. The plane is balanced over the wing box and main gear, with a slight forward bias to put enough weight on the nose gear for steering. In this case the plane has substantial load on this beam from its own weignt and the heavy cargo. If the nosewheel has say 7 percent of total weight (does someone know the weight distributions on this plane, loaded and empty?), just on the back of an envelope I'd say the cracking of the fuselage over the wing box (due to fire weakening of the structure) would increase the loading on the nose gear by at least three times. So this has to be somewhere between sinking into soft ground (on or off runway seems unclear as yet) and collapse.
In short, I don't think as yet there is any reason to conclude that the pilots crashed the plane into the runway, from what we see in the pictures.
OE