Sorry junior physiotherapist for getting back to you so late. Lost this thread somehow. Side bend & rotations are said to be same movements in different planes in c spine. When you check out the saggital plane the movement testing done is - retraction, retraction with extension, & protrusion. The interpretations would lead you to upper C spine or lower C spine. For frontal plane in unloaded position, sidebent would be difficult to assess if head is on the table. hence check the rotation. While the patient is loaded that is in sitting the frontal plane movements of sidebend & rotations could be assessed. This again leads you to either Upper C spine or lower C spine.
Mckenzie is not only for disc lesions, nor is it only extension. If the patient is a yellow flags candidate, they will surely score high on the questionaire. It is unlikely in this patient. I would prefer to repeat that the cervical symptoms would most likely be due to poor stabilization of her back. A cervical assesment could however be done.