Your talking about 100 floors... how much you think that weights? Oh, and the film showed ~10 seconds of fall time, were the estimated equation got 9secs... kinda close don't you think? Falling fast? Not really. Look at other collapses, they all fall in seconds too.
The explosions are the buckling of concrete and steel. As it breaks, it creates the explosions that they said they heard, and what you saw on the slow motion images.
Speaking of the lower levels... the building's lower 8 floors were said to have had the windows blow out. The twin towers, when hit with a traveling object, of such size as the 757's, swayed to such a degree that the glass in the lower levels busted. I remember hearing a person in one floor say that when the plane it, he didin't think that the building would stop it's initial sway (it went that far).