I'm a little limited on time today, so I'm going to only address one issue - the burning jet fuel. Yes, the jet fuel burns at 210 C, but there is no reason to believe that the fire was limited to that temperature. Remember that this is the minimum temperature for ignition, and it will certainly burn at higher temperatures. In an enclosed space with the right flow of oxygen, a wood fire can burn much hotter than 451 F (which is what I remember the ignition point of wood without looking it up). The heat from the fire rose upwards and did what, disapeared? The temperature of the flames themselves would have been higher than that fuel source
http://www.doctorfire.com/flametmp.html
so the high temperature number that we need to look at is that of the flame, not the burning substance. A typical house fire involves wood burning in one way or another. Given that wood has an ignition temperature of 451 F (thank you Ray Bradbury), your logic would make it impossible for the average house fire temperature to be 1100 F.
http://www.health.state.ok.us/progra...ouse_fires.htm
There's also the fact that there were a lot of other things on fire besides the jet fuel. Looking out my office door, I can see a copier (plastic, paper and toner), desk (wood), chair (plastic and fabric), ceiling tiles and cubicle walls (some sort of composite plastic). Remember that just because things are fire resistive doesn't mean that they won't burn. It's a well known fact in insurance that non-combustable buildings will in fact burn, possibly to the ground given enough fuel and lack of firefighting. All the surrounding materials would have added their own flames and heat, which would have gathered in the core of the building, making the steel less and less rigid until it caused more load than it transfered and brought down the concrete.
One other note - while the WTC may have been designed to withstand a 707 impact, that doesn't mean that it was built that way. Buildings, in my experience, are rarely built exactly to specification, and contractors take lots of shortcuts to save money and time. Using slightly less fire-proofing or a slightly weaker concrete mix on the upper levels wouldn't impact the day-to-day survivability of the building (as seen by the 93 bombing), but they would be fatal in these circumstances. Who has proof on how the buildings were actually built, not how they were intended to be built?