my opinion dear colleague, is that if there was a true NEUROLOGICAL lesion (either on cervical roots or on brachial plexus) there would be true NEUROLOGICAL signs and syptoms....This means that your client should have paresthesia in a more peripheral site (which she doesnt have as u mentioned) and some form of weakness in muscles of upper limb (especially if she has more severe clinical presentation)...And speaking about Brachial Plexus lesions, they more often compromise A8-T1 roots...If there are respective signs and syptoms due to those roots then u should consider that neurological tissue is pathological involved

P.S. the aggravating factor u mentioned (reaching the back seat of the car) is true CERVICAL-induced...She probably rotated her neck for that