1. " having the steel conveniently cut up into regular sized sections."

It was broken into pieces depending on the structural use- beams are MADE certain lengths, and all of them in the same place in the collapsing structure are LIKELY to break at the same point.

2. Only the steel which was in places that were insulated would melt- not all had sufficient heat.

3. You do not take into account any of the heat produce by exploding transformers ( when the loads are shorted by arircaft slicing through, or floors collapsing) or any other source.

Like the famous "bumblebees can't fly" proof, the proof is VALID ( as opposed to true) since the movement of the wings was not taken into account. A dead bumblebee CAN'T fly.

You are making the mistake of looking at a single item ( temperature of burning fuel) and NOT at the entire situation. It is not burning in isolation: why should it act as if it was?