Testing a wide range of diseases will be extremely expensive and time consuming. Why spend the very limited resources we have on developing the nanoneedle as a diagnostic test that is possibly equally likely to show you have a psychiatric illness versus a physical one, when you can spend a lot less money and more importantly time to use the nanoneedle as an analytical tool to find an actual ME/CFS biomarker. Especially when that biomarker could lead to a treatment.
If you look at Ron Davis's son Whitney he passes every standard test i.e. he's not ill according to those tests. I've spoken to people/their families who've been told by doctors that there is nothing wrong with them/their family member. One consequence of this is that it reduced funding i.e. since ME is not a biomedical disease. So even at a purely selfish level, i.e. to increase funding for research, the development of a diagnostic test (using the nano-needle) may be in the interest of those with ME/their families.
My understanding is that there is a commonly held view, among medical professionals, that psychiatric illnesses do not have a biomedical/biochemical basis. E.g. the discovery of an autoimmune form of NMDAR encephalitis, which responds to immunotherapy, which has the symptoms of schizophrenia was treated as an amazing discovery. ME is of course viewed in the same way.
I think you're correct about the identification of the compound causing the change in cellular respiration i.e. it would be a very good biomarker (can't think of a better one). E.g. you could use the levels of that compound to assess drugs. So yes it's worth pursuing; however, I think the use of the nano-needle as a diagnostic test could be a game changer.