Before I get into it, I want to echo
@ljimbo423's sentiment and say I respect other's views here and don't dismiss them as impossible, we're all just guessing...
The problem I have is that everything mentioned is speculation with no proof. I can't help but be reminded of Occam's razor...by considering it a single cause we're valuing an unknown, undetectable and inherently complex explanation over any evidence to the contrary. There is also an understandable bias/wishful thinking involved in the expectation that some switch can be simply flipped and cure everyone.
I consider it unlikely that PEM can be caused by several completely different core dysfunctions.
Its my poorly defined and simplistic proposition but lets consider that core issue is Autonomic Dysfunction. I mentioned the structural cervical issues (CCI, AAI etc.) earlier that present with symptoms meeting ME/CFS criteria, these issues involve an explainable cause of autonomic dysfunction & feature PEM. We also have Primary Biliary Cirrhosis, in which Julia Newton sees similarities with ME/CFS, i.e. demonstratable PEM & ANS issues. Then there's a variety of AI diseases that feature PEM, more commonly when they involve SFN and thus impact the ANS (Sjogren's, APS etc.). The common denominator here seems to be impairing the autonomic nervous system.
Most ME cases should have some common dysfunction--a cellular abnormality or whatever--which could be treated. If that is treated, the sensitivity to triggers or modifiers would no longer be an issue.
Relating that logic to the above cases: the common dysfunction is the ANS involvement, the triggers are the underlying structural/immune issues. If you somehow treat the ANS and stop it malfunctioning, will your structural spine problem go away? Would your liver stop failing? Would auto-antibodies reduce and IEFND increase?
Its easy to dismiss these examples as misdiagnoses and not representative of 'real' ME/CFS, but when the NIH CFS study dismissed 1/3 of its subjects in the first week because they found their very carefully selected CFS patients actually turned out to have dissimilar rare diseases....surely that makes the possibility for complete heterogeneity a more plausible idea?