Q1Sorry, so you're saying that the core DID in fact suffer massive dammage from falling debris? (edit: Since you fail to make any mention i'd just like to make sure we're all clear on this issue)
Welded joints are not as tough as single sections of steel. They're not. Now, as to your assertion that the central spire would not have fallen without explosives you're running into massive problems.
Explosives would have to withstand all the debris falling down, enough to rip these massive welded steel beams apart, like fuck that's going to happen
There is no need to do so, everything was already rubble
There is no evidence of it, indeed in terms of timing it shows your earlier sources of explosions to be utterly irrelvant.