I read your intro post and we have some similarities and some opposites. I also have a lot of arm weakness. My EMG was postitive and my binding antibodies were normal. I am waiting for the blocking and other tests. Already my neuro has said that I will be difficult to diagnose. I also have Hashimoto's.
Based on what I have read, I don't think they can absolutely rule you out as yet if you are symptomatic but you probably need to keep looking for other reasons for your arm weakness. Just don't let them stop looking for answers - keep pressing until you know why. I am in the testing phase now. I am on my way today for the chest MRI to look at my thymus.