But anyhow, if Wahhabists, whom the S.A. are vainly trying to pay off, should in turn make plans in S.A.’s sight to attack the USA... why still would they not want to warn the USA, their best customer. It seems all downside for them.
You seem to think the house of Saud is one unified entity, it is not. There are thousands upon thousands of members in the house of Saud, filling all sorts of official roles within the government.
They do not March lock step.
If any of those in official power knew and directly financed the 9/11 attacks, that does not mean that all the others knew.. And if they did to stop it would mean they must turn on their own family, perhaps a more powerful member of the family. Also if they turned against the Wahabbists and started to arrest them, then they would likely turn their violence directly against the hous of Saud.
There is a lot more to the political calculus than, oh the US is a big customer.. A lot more.