Actually there are two responses re: lower down question.
1. They tried in 1993 lower down and the US provided a good analysis of why that didn't work.
I'm sure they read it.
2. If I recall correctly, the building architect said on Sep 12 or 13 that the structure was weaker higher up because it could be.
By the way, that architect was asked by some jack**s reporter
why didn't the buildings stand up?
He answered why did they stand for as long as they did?
That singular fact probably saved 20,000 lives.
The object in question is more if that was the intent, and if they knew that. Personally I think whoever is responsible got more of a result than they ever expected.
How is the structure weaker at the top? The only explanation I can think of is the asbestos issue, which I don't know to be true or not. Would there be extra support columns in the lower portions that did not extend to the higher leves?