Cushings is rare in children but she is almost an adult, the random cortisol is very very high, work only with an endocrinologist and if possible a pediatric endocrinologist. See what the overnight study shows, this will tell us if the brain is making the driving hormone or if it is coming from her adrenal gland itself, then they can decide about imaging her adrenal with an ultrasound
Be sure that they repeat her labs also, 3 separate urines for free cortisols will help, this is done over 24 hours for each collection. I hope this turns out well.
I have a 13 yr. old with most of what you have said and more. he gained 50 lbs. from Nov.07 to Jun 08. He was having the stretch marks also. His eye Dr. told his Neuro to give him Diamox for a possible pseudo Tumor Cerebri which affects overweight teens. he was not overweight when it started. He lost 17 lbs. in 2 wks. It was swelling. He had good eye sight but when they did a vision field test it came out abnormal for peripheal vion. With normal eye test. Then in 3 wks was 75% worse on Vision file test again. Gave him Diamox and ended up doing IV steroids to get inflammation in body and optic nerves.
Had severe clonus is how it all started in Nov. 07. The psuedo tumor is a moch tumor that causes alot off migraines. Also they just checked his blood gases and plasma for acidiosis and Ammonia. Found his Ammonia levels to be 71 when 33 is normal. this causes poor cognitive, concentration and many other things. We did Cushings and was normal. This may not help you but It might be something that clicks for you.
Also I would like to know what is MG?
They still cannot find the underlying diagnosis on my son but the Psudoe Tumor was the swelling, stretch marks.
I am not a Dr. and my story may not help but it is a work up to look into.
They did a blood test before during and after exercise to find out about the ammonia levels and other tests. Lots at once. maybe this will give you alot more answers to rule out. They had said CFS on him and are finding out it is Metabolic.
I Hope this helps in some way for your daughter. I know how it feels. Been there. Am there.
Good luck and God Bless!