ttowne 1267. Good call on the fuselage ignition source being friction, but if you look at the #2 engine, there is a lot of scorching on the fuselage around the ehaust and it is still attached by a solid-looking umbilical. If it was still getting fuel..... Well we saw with AF in Honolulu and more recently QF in SIN how difficult they are to stop (of which I normally strongly approve).
I've flown many times into SFO on 747 2/3/400, from the North, normally the Bay approach to RH visual cleared visual relatively high, and if you are not ready for getting into the descent fast, it's very easy to get into the high rate/low thrust area close in. 300' per mile has always worked for me and we were also taught on all the A/C I have flown (F27 and up) to know your stabilised 3' ROD on final.
Some good stuff on this post, the inspectors shoul be able to get the recorders, so all will be revealed.