My Wylie has Cushings....very complicated disease and many general vets don't test for it correctly, nor treat it correctly - so keep that in mind.
As I started reading your description, Cushings popped right into my mind. From what you said about the tests, I don't see that they truly tested for Cushings.
If Cushings is ever suspected in a dog, the first thing to do is Cortisol-Creatinine Ratio Urine Test. The test does not *confirm* Cushings. It can tell you whether you should test further for Cushings....the reason they use this test first is bc it's cheaper than going straight to ACTH testing. I would def get this urine test.
From there, if the test showed concern - you'd do an ACTH test, and usually they do an ulstrasound the same day, to look at the adrenals.
It's a very complicated disease, both to understand and to treat...but take it from me, it can be done. Testing and treatment titering can take a long time, so be prepared if that happens. The U of Tenn can help if they think the ACTH points to Atypical Cushings -- U of Tenn is really the only place that can properly diagnose Atypicals.
This is my fave Cushings Overview link, it helped me a lot:
Cushings (read the symptom list, some may apply to your kiddo, but not all)
Hang in there!