An ACTH test may work on this horse as she appears to have been Cushingoid for some time now. I would opt for Permax at a beginning dose of 1mg daily. It would be wonderful if dietary management would work, but you simply don't have the time at this juncture. Cushingoid horses can be very painful without rotation. The pain seems to originate from deep flexor tendon pull and is often not evident on the sole, especially if the horse has good hoof mass.
Blood glucose is nearly always normal or only slightly elevated.
Horses utilize volatile fatty acids along with glucose for energy and have lower glucose values than humans or many other domestic animals. Cattle are even lower. It isn't a very helpful test in the equine. You will need to change to a high protein, high fat, low carbohydrate feed. A short course of antibiotics may also be of benefit to lower the Streptococcal organisms in the cecum.