Well i hope you get other experienced people help . Just want to say that
76* is not cold water at all -it perfect temperature for bettas. My bettas are really old from age 2 years old to 5 years old live in that temperature and some of them even lower temperature
1/2tbsp which is 3tsp/gall of aq salt the dose that on the package it not therapeutic dose -it just for preventative care. When you treat bettas with aq salt you need 1-3tsp/gall . Usually when you do aq salt you qt fish in smaller container and do daily 100% water changes. I believe in cycled tank you can just do 50% daily water changes. As you probably know you can use salt up to 14days.
But you other fish might not tolerant to that dosage .
I hope it not fungus because if it true fungus will progress fast and you really need to treat him , you right not to wait. But if it not water issue then why he would have fungus .