If you are going to use a heater, try to find a 25w heater that you can set the temp at a steady 78. I use mainly 1g bowls, so they are not heated, I just make sure the house is warm for them.
Bettafix will help speed the healing. My VT Dopey tore his fins a long time ago, maybe 4 months, maybe longer. They are just about done growing back, but I havent' used anything in his tank to speed up the process, just clean water.
As for the biting, some bettas bite because they are overstimulated, others bite because they are understimulated or bored. The only way to curb the biting is to figure out what is bothering him and try to remedy it.
I had a tail biter that bit because he could see his neighbour, and he would get worked up and think his tail was his neighbour. He would bite his tail in his excitement. I moved him to where he couldn't see a neighbour and the tail biting stopped.
I had another tail biter that was bored and bit his tail to occupy his time. I added some "toys" to his tank, and moved him to where he could see other bettas. He has since stopped biting.