Earlier quoted context omitted.
I'm not sure how those things discredit the janitor. Are you as critical of the politicians, who have built their livelihoods off of going around speaking to corporate interests? What about the government officials who put out the 9/11 commission report, and were paid for it. Are they similarly implicated by the act of self promotion? Or does that distinction only apply to the janitors of the world?
> I'm not sure how those things discredit the janitor. He claims there was a massive explosion that shook the walls, caused part of the ceiling to cave in, and other significant effects. The obvious source is the plane impact. If there were multiple explosions of the magnitude he claims, surely other people would have noted them. And of course, the standard conspiracy theory caveats apply: Do you really believe that…
The conspiracy theory about explosives is even more idiotic than that. It's not merely that the conspiracy would needed to have been extremely complicated and involve 10s or 100s of silent accomplices, which is true. It's also that it would have been completely unnecessary and redundant, and is therefore a violation of Occam's Razor.
Whatever the purported political objectives behind a conspiracy to put explosives in the towers, the planes themselves should have been sufficient to achieve those objectives, and so what motivation would there be to tack on yet another entirely different conspiracy (different in terms of operational details, skills required, additional headcount, etc)? You're multiplying the difficulty and complexity of the operation and increasing the chances of getting caught by a good 2-10x, for little benefit due to that redundancy.
> Common sense discredits the story.
They say things like "jet fuel can't melt steel", when nobody even makes that claim. The claim is that the burning jet fuel made the steel weaker. So don't expect them to exercise common sense when they're insisting on these strawmen.