I can't imagine that fatigue is not the most debilitating part of all this for most folks. It sure is for me. PEM is very real in my case.
I am also interested in your view that ME and CFS are not the same syndrome. Most of the science I read has them as the same condition.
Fatigue and energy insufficiency are not the same. What we have is energy insufficiency, and a ceiling on energy production that is very low in some cases. So on demand ... there is not much more energy we can make, and we switch to rely on glycolysis.
When we use the term fatigue we conflate it with all the other types and definitions of fatigue. Everyone gets fatigue, ergo we just have to push through it.
Mild, well managed patients might not experience chronic fatigue. Many ME patients do not experience chronic fatigue. Byron Hyde has seen many patients with no chronic fatigue, but with all the other things. I have experienced it myself, and it was a surprise ... but it was not remission, I still could not function and even could get PEM.
I am not sure I would claim that CFS and ME are not the same syndrome, at least in all cases. I will most definitely claim that the patient cohorts selected under various definitions are different. Look at numbers alone ... prevalence of 0.2% under strict ME, prevalence cited as high as 7% under very weak CFS definitions. That's a huge gap, and allows for potentially huge selection bias.
So right now we can probably treat them as similar if not the same at a clinical level, but we cannot be sure about the full patient range, and that will change. Fukuda, for example, does not require PEM but it is an optional symptom. Oxford, as the worst definition, does not require any symptom other than chronic fatigue, though with caveats. Most weak definitions seem to point to about 2%, but strong definitions are 0.4% or under. When you have such large differences you are almost sure to have selection biases.
PEM is much more specific, but we still lack good operationalisation, largely because we do not use any objective measures in most studies. There are now multiple tests still being investigated, PEM might be associated with energy deficits, cytokines and other blood markers. This does need more research. When we start using those we might be more able to get good cohorts or better measure clinical outcomes.
Now in MS they have exercise intolerance, but a study using CPET showed their response is very different to ME. Its exercise intolerance, not PEM.
For most, for now, any clinical presentation is about treating symptoms, and its very individual, so it does not matter hugely, from that perspective, how you define it. For management considerations, including activity planning, it might make a difference though. With exercise it makes a huge difference if the person does not have PEM. Current cutting edge research is looking at flexibility and strength training with no aerobic component, as aerobic metabolism in ME is very broken.