How about looking at it from the other way round? How do you know it's compression at the brachial plexus when the imaging shows abnormalities at the nerve root?

I'm in a similar situation with a C7 disc bulge with corresponding reduced nerve conductivity. I think it is this that is causing most of the pain and the TOS is a by product of very tight scalenes trying to support the irritated joint.

TOS disgnoses are in many cases the result of what's left after having investigated all other possible causes.