destruction times

How long is it going to fall?

Nearly as fast as it would fall through air?

Can an object fall through mass 5 times greater than itself nearly as fast as it would fall through air? (When only force available is gravity?)

You need to be no expert in physics or structural engineering to answer this question. And you also need no expert to tell you what the answer is. All you need is a little bit of common sense.

Undamaged part of the WTC1 under the impact zone was approximately 5/6 of whole tower. Photos of impact zone of WTC1 can be found here to check that there is only 1/6 of the tower mass above the impact zone. Use sky lobby as a reference point. See also this picture and this video.