First - just because an item will ignite at 500 degrees does not mean it will only produce that much heat. Try dumping some water on an oil fire and watch how the heat goes up. The fire control systems in the buildings most likely did just that. The number needed here is just how much heat could a jet fuel fire produce and what would be need to make it produce that.
Second - The steel did not have melt to collapse it only had to soften. And in support of that the building that fell first was the second one hit. The beams at the second crash point were bearing much more weight. Those beams would be the most stressed and likely to sheer first. A good study would be to ascertain the temperature that would weaken the steel to cause the connections (rivets, bolts, or what was used sheer off and start the collapse.
The third building? There's another whole other problem.
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