I think the bankrolling of 9/11 by the Saudis is almost commonly understood. Personally, I think that is why you saw the current head of the House of Saud purge all the old guard and even put some of them in jail. I have speculated that Trump would not meet with them unless they cleaned things up and removed those scoundrels, which they did.
#4 I read years ago that some of the plotters were killed by leaving them in the desert and saying that their car had broken down.