Gas Burners are made of alloys & cast iron. They are beneath the flame and in fact do not even come into contact with the flame. The hottest part of the flame is up around the tip of the blue part. The burner just never gets hot enough to melt even when the skillet is over top. However, were you to force some air into the burners, you'd have a different story.
Electric burners probably would melt if it wasn't for thermostats and fuses. Again, they don't get hot enough.
The twin towers lost their integrity when the planes flew into them. The flames were fed by jet fuel and fanned by updraughts in much the same way as your wood stove is when the door/damper is left partially open. I'm sure that the climate control system in each of these buildings, which alters the air pressure within, played some part but have not heard anything on this.
Steel softens when intense heat is applied. That's why the blacksmith uses a blast furnace rather than a garden hose. Not rocket science by any means.
It would have been a miracle if either building had remained standing.
As for conspiracies... From what I've seen in the past couple of weeks, I would have no problem accepting that your government knew of a plot but not the specifics. Given their proven disregard for human life, I would accept that they would allow an act of terror as an excuse to raid Afghanistan, Iraq and whoever is next on the list.
I cannot imagine for one second though that they would have allowed 911, had they known the magnitude of the event.