In general, heat is a much higher risk to your chickens than cold. Chickens do not need or benefit from supplemental winter heat, with the exception of risk of frostbite to large-combed breeds as I noted. Adding heat is not doing your birds any favor. They are much more apt to suffer and die of overheating than cold. If they are panting and lifting their wings up, they are getting too hot, and a shut-up coop in summertime (and in winter, to clear out moisture) needs excellent ventilation. 1 square foot of ventilation per chicken is recommended. They may tend to snuggle up together on the roost regardless of coop size.
I don't know what your climate is, but I run a fan for mine in the coop when temps are in the mid-80s and above. At temperatures higher than 95 / humid, they can really suffer and be at serious risk without adequate shade and cooling. They are perfectly comfortable at winter temps in 20s and 30s, and even teens, and people keep many winter hardy breeds happily at lower temps. Think of your chickens like cows or horses. They don't need ski coats!
It sounds like you have done a lot to try to resolve the illness. Maybe post in the Diseases sub forum to try to get some more help? Good luck. I'd make sure your coop has enough ventilation and that temperatures stay comfortable in the summer.