@crypt0cu1t I will never diagnose anyone or tell them that they do or do not have ME/CFS (or any other illness) but my gut (which could certainly be wrong) from talking to you in many threads and via PM is that you do not have ME/CFS.
I was given a "CFS" diagnosis (b/c "ME" is not used in the US) by five doctors in 2013 and 2014 and I whole-heartedly believed it was my diagnosis when I joined PR. I volunteered with OMF and the End ME/CFS Project (in 2014) and read everything that I could to learn more about my diagnosis. I matched in many ways (b/c it started as a post-viral fatigue syndrome following Mono from EBV, I had positive IgM+ titers for EBV for 3-4 years post-Mono, I had very low NK cell functioning, non-existent vasopressin, and matched on all of the things that were used in lieu of having a bio-marker).
But when I saw an ME/CFS specialist, as time passed, we both realized that I was very different from his other patients on many factors and the biggest factors were that I did not have delayed PEM, flu-like symptoms, or brain fog. I did however have POTS & MCAS (two of the biggest co-morbidities in his patients). I also had severe muscle fatiguability and muscle weakness (like his other patients) that led to me having to use a wheelchair but mine was progressive and affected my lungs, diaphragm, and breathing.
He said that my muscle and breathing weakness was more severe than any of this other patients (meaning those who could come to his office and not those who were 100% bed-bound which I never was). I failed every test of hand grip strength and spirometry/PFT testing of lung functioning (and this was different than his ME/CFS patients who could pass those tests in the moment but then had delayed PEM).
We knew I had (confirmed) Hashimoto's Disease, POTS & MCAS but could not figure out the missing piece until I tested positive for the N-type Calcium Channel (or LEMS) autoantibody. And then once high dose IVIG and Rituximab brought about this "remission" (for lack of a better word b/c I am not at pre-illness levels), we knew my case was autoimmune. If you have the myasthenia gravis autoantibody at 11x normal plus the Sjogrens autoantibodies, to me, your case is autoimmune.
Does this mean that you also have ME/CFS... I truly do not know? Did I have it in the beginning and then it morphed into other things... I also do not know? My fatigue was severe in the beginning and I had severe insomnia and GI symptoms but all of that shifted into POTS & autonomic problems, muscle & breathing issues, and then MCAS with anaphylaxis. It was challenging to tease it apart but once I tested positive for 11 auto-antibodies, there really was no other logical plan but treatments for autoimmunity.
I am also baffled why you have symptoms of autoimmune encephalitis but test negative for the autoantibodies but there are cases of everything that are sero-negative and you have enough other autoantibodies, plus a positive response to IVIG, that I would continue in that direction (if it was me) and worry less about the label even though it is incredibly frustrating not to have one!
Back in 2015 & 2016, I had people tell me that 100% I had ME/CFS but was in denial and needed to stop searching for other illnesses and at the same time I had people telling me that I did not match with ME/CFS and that I MUST keep searching. So you will get a variety of feedback and all you can do is research everything and work with your doctors to find the right plan for you.