First of all, I would like to point out that the collapse of the World Trade Center was a surprise for everybody, including the planners of the event. Had a complete demolition been expected, the impacts would have occurred later in the day, likely just after lunch at around 1 PM. They would also have timed the strike to be more simultaneous.

Now, taking down any tower will require that explosive charges be set at the main supports. For the Twins, these supports were the central beams. The impact of the airliners, at 400+ mph, which allowed them to reach these beams even as they exploded, initiated the collapse, particularly with the second impact, which was nearly dead-center. The ensuing fire, fueled by a plane that had just had its tanks topped off and contained in the structure of the Tower for maximum heat, did the rest.

As for entropy, had the day come when the WTC had to be dismantled--and I can't think why this would happen--it most likely would have been dismantled from the top down piece by piece.

To a certain level, yes, thereby minimizing collateral damage. The process, though, would have been a contractual and engineering nightmare.

The buildings were designed in such a way that demolition would require explosive charges set at least half way up the Towers’ remaining structure, but not so high that there would not be enough weight falling to bring the rest of the structure down.

Yes, buildings must eventually be demolished.

Yes, they are designed so they can be demolished.

No, there is no way the mass of the Towers would have ever allowed them to remain intact so they could fall over like a felled tree. A lean of greater than a few degrees (I put no number here, because I don’t know what that limit may have been) would have snapped the central supports, resulting in the same vertical collapse.

Dan