Well I'll be the first to say your tank is overstocked with that molly. It's completely possible that your molly ripped his fin off as well, although not probable, it's still possible.
Ballon molly's can get pretty big so either you get a bigger tank or it's going to have to go, I'm sorry to burst your bubble.
Otherwise, good for doing a change every 3 days, you can keep it at twice weekly though with 50%, make sure to siphon out gravel and stuff. Also changing to a higher quality diet will help initiate the healing process, any frozen foods (not freeze-dried!) will be good as well as either getting Omega One or New Life Spectrum Betta Pellets.
Also you should look into getting some Stress Coat Conditioner, it is a conditioner so you won't have to use your other one while using it but it contains Aloe which will help heal his fin up quicker. Although this is probably going to be a slow process, it's best to just keep a very close eye on him from here on out. Watch out for black edging, anything fuzzy or out of the normal looking, alright?
Also, if your heater is adjustable, it would be best to put it right up to 80, although do it over about two days or so, so you don't fry your fish lol. Betta's prefer around 78-80 degrees, anything lower than 75-76 isn't good and your just teetering on the edge. If it's not possible to adjust it, is it possible to heat your room a little more?