I'd be looking of the dynamic scapula stability and strength of the rhomboids, lat dorsi and checking the levator scap for tightness. Assess the shoulder especially looking for limitation of internal rotation in the "catch" position ie above 90deg shoulder abduction.
If shoulder is unstable or weak or tight ( or a combination of these) then the swimmer will compensate with thoracic spine.
I would put the patient in prone and the arm over the edge of the bed to assess functional movements to see whether it is the positions or the active movement that hurts.
A trial treatment to the thoracic spine and ribs would be a reasonable start as these would allow a quick improvement if the problem is thoracic mobility.
I suppose my answer is based on practicalities rather than answering an exam question!
Let us know what you find!