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 the Wikipedia reference-linkspondylosis.
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.