Most average drs aren't really "up" on TOS, or they have a bias against it from minimal knowledge on the subject.

The MRI pretty much will only show if the c spine is or isn't the cause of the arm pain or if there is a extra cervical rib or large bony structure causing the compressions. It doesn't show TOS itself as it can be a soft tissue problem.

You might look thru our stickys on the TOS forum- the second to last post has a "our symptoms " thread link that might help you see what many of us actually have symptom wise - the websites about TOS don't list nearly all of them.
But what you wrote about your arm/ neck pain sounded very familiar.

we have a TOS drs list sticky too.

I had the best help from an advanced PT guy and the very good chiro that I was lucky to find.