What an ordeal you are going through!
Seems like you've had an enormous amount of tests and there's still a lot of head scratching.
Having exhausted any new ideas from both cardiologist and GP, with high cortisol, I would now take myself to an endocrinologist. Even though your GP has ruled out Cushing's, it doesn't always follow that weight gain is a "must have" sign.
Not that I'm saying you have Cushing's but endos are the experts when it comes to knowledge of our hormones, cortisol of course being one of them.
Worth a shot, don't you think?