Creative Speculation

> They assume that the top piece imparts all its force onto the lower part at one single instant, as a rigid monolithic object. This assumption is clearly not true and it clearly goes in favor of collapse of the idealized model, not against collapse.

If you believe that, then here is the specific part of Bazant's argument that you need to address:

As I said, you can disagree all you like, but you can't use your own contradiction as proof that "Bazant has no argument that needs refuting." Yes he does, and you haven't done so.

In order to halt the collapse, all of the kinetic energy of the falling mass needs to be absorbed or dissipated, so Bazant and Zhou take the crushing of all the columns on one floor as the limiting case that absorbs the most energy. If you know how the structure could absorb more energy than that in that initial collision, you haven't shared it. Their calculations then show that the total kinetic energy available in the top block was 8.4 times more than the column could resist. (And that, by the way, is irrespective of the "equal and opposite reaction" in the top block.) That calculation does not "assume that the top piece imparts all its force onto the lower part at one single instant, as a rigid monolithic object." Rather, it shows how much excess energy must go somewhere if the collapse is to be halted. Hand-waving doesn't make it disappear.