However, speaking of irrelevancy, I respectfully disagree that the third point is so, in fact, it may be more relevant to the discussion than the other two. The issue isn't whether the U.S. would allow Saddam (or any other external opposition, whoever that might be) to march in, although I would have to agree with you that we probably wouldn't, it's whether the U.S. will protect the current Saudi government from their internal opposition. Here it sort of depends on whom the internal opposition is - if it's merely another faction of the current ruling family, possibly not, if it's a fundamentalist faction supported by the Wahabbis, intent on turning Saudi Arabia into an Islamic republic, then things get very interesting and all bets are off. In that case, too, the ostensible deployment of al Qaeda resources in Iraq may have little to do with Saddam at all...
Here speculation gets a little wild and I'd better stop. If it reaches this point we're in for some real instability in a very sensitive region - the Saudi calculation will be, as it always has been, how much we are willing to cede them in favor of stability. If they can no longer offer stability, and I suspect that may be increasingly the case, then this "friendship" everyone seems to be referring to will show itself to be the realpolitik it really is.
With Saddam in power Iraq is weak economically (little or no foriegn investment, not selling any oil, etc.), it poses little or no military threat to the Saudi's(U.S. protection), and Saddam distracts the rest of the world's attention from Saudi Arabia.
If Saddam is replaced the new government will most likely comply with the U.N. resolutions and thereby be able to sell oil. This will lower oil revenues for the Saudi's, who are already running a deficit. If the new government is a democracy it would be a natural ally for the U.S. and this would greatly diminish the Saudi's importance.
A democracy in Iraq would pose other threats to the Saudi's as well. The Saudi people may start to demand the same freedoms that would undoubtably be enjoyed by Iraqi's living in a democracy. A democracy in Iraq would most likely stregthen the Wahabbis in Saudi Arabia as well because it would add fuel to their anti-West fire.
As far as them funding fundamentalists in Iraq; it seems they may be trying to replace Saddam with someone that would be equally hostile to the U.S., thereby, in effect, maintaining the staus quo.
My point is that the Saudi rulers will be better able to defeat their internal foes with Saddam, or someone like him, in power.