Here's an update.
My vet was out again today (Yes, on the 4th of July) and she consulted with the specialists (pulmonologist and cardiologist) at the equine hospital at a local university. They do not think my horse has heaves, thank God! And this is the official opinion of 4 veterinarians who all got together to review and discuss Khaiyaann's condition. They said that he would have a lot of wheezing in his lungs if it was COPD, but during the last 4 exams he had no wheezing at all.
They think he went into congestive heart failure due to pneumonia. I guess he had a lingering low grade respiratory infection ever since I transported him, and it just got worse and worse over time. I wish the first vet who treated him would have diagnosed him with pneumonia instead of COPD, and started him on more aggressive antibiotic therapy - it would have saved my big boy a potentially life threatening crisis, and saved me a lot of money! I even called her 3 days into his therapy and questioned his poor response, and she told me that "it'll just take time, so relax." What really makes me upset is that, when the first vet examined my big boy, he had crackles throughout his lungs, and some minor/occasional wheezes. She said the crackles weren't indicative of pneumonia.
Well, now 4 other vets say that crackles ARE indicative of fluid in the lungs and are associated with pneumonia or pulmonary edema, NOT with COPD/heaves.
The good news is that the pneumonia was pushing him into fluid overload and hurting his heart, and now that we're on the right track treating his pneumonia AND his heart condition, he is steadily improving!!! He will be on antibiotics for weeks to come, but that's ok. He was galloping through the pasture today just as happy as could be!!!
As for his heart murmur at grade 5/6... they say that's not good. :( BUT he is responding very well to the lasix and the enalapril. When his antibiotic therapy is finished we're going to reassess the murmur.