Any holistic vets in your area? has horse been treated naturopathically? acupuncture? equine therapy? (weekly, then biweekly, then monthly etc)
Riding? it all depends - horse should be walking everyday (does she self exercise in turn out or is she mostly sedentary).
With all that you've posted about this horse, she just seems "unwell" to me, I'd be inclined to just retire her to permanent pasture puff status.