1. I desperately need the OST, it's honestly such a banger.
2. Will there be any special interactions or anything in the future if we return to the island after fighting e6?
The OST is mostly from Eric Matyas. He has created dozens of free to use music tracks. Just look up his name and you should find most of the music in Feline Fantasies, but if you want any specific you can't find, just holler and I'll see if I can provide them.
The next update (and subsequently the final one) will feature new dialogue for all the old felines after beating the game. So you'll get those special interactions, plus two entirely new felines to round everything up.