In that workload with a saddle that didn't fit, on a back that is still developing, I would not be shocked at all if the saddle was the only issue. Sorry if I missed it, but what points to arthritis specifically vs soft tissue/muscle and joint soreness? I would be getting frequent body work done (or learn the basics of massage and do it yourself, perhaps between professional sessions), and would keep her moving myself without a saddle or rider on her back and not on a circle (so handwalking or ground driving not in a circle). Arthritis or plain soreness will benefit from gentle movement, but I'd have a hard time believing it is arthritis without seeing the radiographs (which I personally proabably would not spend the money on yet until I treated it like soreness from an ill fitting saddle combined with a fairly heavy workload for a youngster). It does sound like you have a good team, so hopefully they can get to the bottom of everything and help her start feeling better.