1) I dont buy your reasons for success because success is not what I forsee for the IS. I see it as a threat that SA will have to face sooner or later. I find your arguments on SA's invulnerability to the effects and influence of IS weak.
2) I never said an all out invasion. That is your conclusion and your mistake of looking at IS as expanding everywhere at once. This is not a scenario for tomorrow, it is one for one or five years unless massive intervention is undertaken to defeat IS now. The rapid force by which IS is recruiting and expanding is not to be taken lightly. My vagueness has little to do with your lack of timeframe. At first the argument was about the unsuitability of the desert for guerilla warfare which a basic read through history proves utterly wrong. Then it was about Airpower, which history has also proven utterly wrong. So which of your arguments about KSA's Iron curtain then stands on factual pillars?
3). Essentially, those events in Syria which were backed by KSA in its short sighted move to reduce Iranian influence has resulting in an ideology that is not much different from the flavour pushed by the KSA state itself. Except that unlike the strict regulation of the ideology in KSA to ensure that no one other than the government has the right to use force, here the ability to twist religion into criminal warcrime has been given a free hand. What stops IS then of basically disguising its propaganda in the guise of being compatible with the KSA state's religious projections?
Again, your focus is on assuming that IS is fighting like some ragtag bandit group when it has displayed much more coherency in its organization. Those 40000+ fighters are not going after KSA tomorrow, they will come for KSA once they are through with Syria and Iraq.
As a counter argument and strategy, it may also be that IS could/should be infiltrated and then funded by KSA intelligence. Since their focus lies much more on the elimination of all other ideologies within Islam except theirs..which means that KSA is easily the most safe from IS..whose greater focus will be on the Shia crescent.