Although you have gotten some mixed opinions on the diagnosis (which I know is frustrating), it seems like the doctors had relatively good news for you.
Unfortunately, the more I read and hear, the more apparent it is that these neurological diseases often remain a mystery - even to the experts in the field.
Hopefully, you can now concentrate on finding and treating the cause of your arm problem - and worry less about some other larger issue. Hopefully your body will take care of itself and heal with time. Be sure to maintain a good diet and maybe some basic supplements for nerve health. That may help your body get the "upper hand" in its battle to heal.
I'm pulling for you !!!