Have you had an echocardiogram? Thats the test that would show cardiomyopathy!
Well, you're young and female, which right off the bat puts you at very little risk for heart disease (assuming your family history is normal).
Your Holter monitor (named after Norman Holter, the inventor) seems to have been OK, too.
Your stress test is, as you say, pretty darn good. Chances are that you wouldn't have been able to perform anywhere near that well if you had any kind of cardiomyopathy, which really limits what you can do physically.
However, since you do have some swelling of the ankles and some nighttime shortness of breath, it wouldn't be a bad idea to get an echo just for a more accurate picture.
You do mention that you have anxiety, and it's worth noting that nocturnal panic attacks can have very dramatic effects, more severe than those that happen during the day. You can google this. If your ticker is OK, something like nocturnal panic might might be contributing to your dreams and drowning sensations.
Sleep apnea can also be a factor.