Epsom is very mild and less stressful than aquarium salt. Epsom usually recommended for fish with constipation, bouyancy problem, internal infection. Epsom also has antibacterial affect so would also help with his fins .
Heaters , i set them for 78 but a few of them heat only up to 74*. I keep a few of my bettas in 74*. The heaters capacity for a bigger tank, so i thought it would overheat rather than bring the temperature down. But so far i have it for about 2 years and temperature is stable. I always unplug the heaters about 5 min before i do the water change.
I am also very paranoid with all heaters and check them daily (3 times a day). I had one that overheated , and i had one that stopped working when the temp in the house at night was 65*. So i always check on them and can't wait until summer, when i just take them out.
It good that he is eating. I hope he will get better . I really think you should try Epsom . Just make sure it's unscented 100% Magnesium sulfate without any additives .