I agree with the above post that it will be very important and revealing to assess the Tsp. Alot of the symptoms sound very sympathetic-y / thoracic-y (sorry...can't think of REAL words!).
If she is habitually sitting with a flexed Tsp as you noted, she is likely extending through her mid-Csp +++ just to be able to look forward therefore closing down the Csp facets and foramen even more than they are just with thespondylosis.
Therefore, check mobility of Tsp (including ribs) and treat appropriately; educate and train re. posture (as much as is possible given anatomical / arthritic constraints); mobilize the Csp as much as possible.