(I wonder if my complete lack of clue about what is plausible in this context is a common theme amongst conspiracy theory believers?)
In truth, I think the ME is a relatively uncoordinated place. All these different motivations and intesrests exist. Thhe pipeline idea is real-ish, the Qatari support for "extremist Islamists" is too, though Saudis support Islamists too when it lines up. So, it's hard to entirety dismiss or disprove any of these overarching motivation theories. The US and Russia have always proxy-fought in the region, but they never really knew what they wanted there.
My thought is that there is no coherent actor, just a mishmash of interests, ideologies and unpredictable chaos.
As anyone who has tried to get anything done on multiple continents knows, any complicated goal (e.g., build a multinational pipeline through remote and wildly insecure areas) will tend to lose out to the interests of locals. A necessary but insufficient condition for success of such goals is to spend enough to make locals' interests coincide with one's own. The difficulties faced by this hypothetical pipeline lead one to doubt its construction would profit in the long run.
The advantage of the USA military-industrial complex is that they actually DGAF what happens as a result of all of our profligate and evil war spending. The spending itself is the goal, and they're just as happy to siphon off a stream of resources supporting ISIS in Syria as off some other stream supporting Kurds in northern Iraq. That's why our international policy (and the media narrative carefully curated to provide cover for it) is so schizophrenic. We believe in democracy!/Down with Morsi!/We DGAF!/War! That is a big conspiracy, but nobody wants to talk about that.
Nicely done.