Couple of things:
Have you tried a Feliway diffuser in the basement? That might help her to feel more at ease.
As for food, have you tried any grain-free canned foods? Any with novel proteins like venison, duck or rabbit? One of the problems with Orijen is the number of protein sources in it, which may not be a good idea for a cat with digestive issues. It contains chicken, turkey, whitefish AND salmon, and fish in particular can be a common allergen. Also, as TeriM mentioned, what about a raw diet? http://www.catnutrition.org/index.php