The 'evidence' doesn't point to shit. What is known is that there were briefing memos that were more analyses than warnings about potential attacks. It's a stretch to say "the FBI and CIA knew about the plot to crash planes on American soil in a massive terrorist attack". The information that they had at the time was far more general than that, much of it was speculation, and, frankly, you're looking at everything with a hindsight mentality. So no, that's not "substantiated".
You know, I wrote a paper about OPEC wanting to abandon the US dollar and how that helped the US fuel its interest in invading Iraq. But to make the leap in logic to say that the US planned or assisted in the destruction of the twin towers and thousands of its citizens so that it could invade Iraq, which took a year and a half to get to, is, again, a really big stretch. Whether or not something presents a "golden opportunity" basically does not mean jack shit in terms of whether or not something actually happened in the manner that you choose to speculate. I'm sure there have been some golden opportunities for you to rape somebody and get away with it, but I'll give you the benefit of the doubt that you didn't actually do it.
I chuckle that you suggest that I am on an extreme end and that you're in the middle. I'm the one that is not making assumptions and following insubstantial evidence and calling it proof. The furthest that it's even reasonable to speculate, let alone believe, based on the evidence that is available, is that the US, to some degree, knew what might happen and failed to prevent it from happening.