You've answered your own question, of course: Doctor's haven't been able to get farther with MS because there are so many variables to the disease, and how it affects individuals. Diet, environment, heredity, virus, bacteria, stress -- all those things and more might .. might..have some bearing on the basic disease process,
which is: our immune system "mistakes" our own myelin for a "foreign invader", and goes about destroying it.
I do think, though, that fatigue is pretty common among most of us, at least here, and on other MS forums I frequent. Sometimes early on, the fatigue will appear, and the person won't relate it to MS, directly, as there aren't any of the other common signs or symptoms.
I think there are some symptoms that are common with people with MS..but as you noted, a lot that differ, depending on the person. And then there is the matter of explaining symptoms -- one person's pain, for example, is "mere discomfort" for someone else -- in other words, a lot of the descriptors of symptoms are subjective, and can be misinterpreted. But there are specific signs that the doctor does look for, both in his physical exam of the patient, and also when conducting the various tests. For example, while lesions can appear on the brain for all sorts of reasons, there are specific areas of the brain that -- if lesions appear there, it is more likely MS than some other cause. Each of the tests for MS, while not specific to MS, do have specific indicators that the doctor looks for, while making his diagnosis.
...I am not a doctor, nor health professional, and don't pretend to be one, here.....