The physicist in the group should be classified as a particle physicist, not a structural mechanics expert. That's as worlds different as saying that someone who studies compiler research is qualified to talk about recurrent neural networks.
Conspiracy theorists (in general) have been known to make stuff up, and non-experts who believe in them can easily parrot this stuff thinking it's true. A great example of this in history is the "Magic Bullet Theory" of JFK's assassination, which holds that the single bullet is false because the bullet would have had to make sharp turns to get to its next target, which is almost entirely based on made-up positions in a fake film.
Thus, for an article which is espousing that the towers came down only via controlled demolition, it is necessary to find independent sources of all facts mentioned, even ones as pedestrian as the expected weight of a floor of a building. (I haven't done the math myself, so I hold no opinion as to whether or not the numbers in the article are correct).
Your characterization of the Magic Bullet Theory is inaccurate. The error is in not realizing the car was of an unusual geometry (I believe JFK was elevated, and the seats were different widths apart between the front and the back- unusual for cars of the time.) So assuming the car was normal, and all the seats were the same distance apart and the same height, you would need a magic bullet. Applying deeper understanding of the setup of the car accounts for that, reducing it to the improbable bullet theory. That doesn't mean they were wrong, that means they didn't have access to the car to do direct measurements. When the government locks down the evidence (or in the case of 9/11 immediately ships it off to china, before the investigation begins, literally shipping buildings across the ocean to be recycled) it's not the fault of the conspiracy theorists if they come up with wrong hypothesizes when denied access to the evidence every legitimate researcher should have access to.
Regarding "fake film" you seem too be referring to Oliver Stones "JFK" which is not the source of the Magic Bullet Theory, or any of the theories there, it is merely a collection of the theories prominent at the time, and then fictionalized.
Yes. "Truther" is code word, like "birther" and "denier" and "conspiracy theorist". It basically means "anyone who doesn't agree with the Official Positions Of The Ministry of Truth, and is thus ipso facto without credibility".
It's from a very anti-intellectual ideology.
Literally these people reject the scientific method. They are rejecting argument evidence and claims based on the source, not based on the quality of the arguments (which is talking to the person not the point, and a form of fallacy of appeal to authority)