I do like cheese lol, and they have vegetarian/vegan cheese as well that's also really yummy.
Live plants do require special treatment and some like sun light more then others. I'm not the one to ask about live plant care as I really only keep Marimo moss balls, which are great to have in a Betta tank.
- the ones for sale here are super tiny though)
API Stress Coat+ is GREAT for Bettas! Many of the users here use have given up their old water conditioner and started using this instead.
If it's dying then it should be removed from the tank. With it rotting in the water it could be what's making your fish ill.
You should easily be able to find some silk plants for him like what are in the attached photo. I found mine at PetCo, but they can also be found at PetSmart and any local fish store. They look good, are soft so they don't tear fins and are easier to take care of then live plants.