“The title of apikores is not about making fun of them; its about knowing whether there is a mitzva to make fun of them, and if I can eat from their kashrus, etc.”
I’m not going to argue with that. Even if you agree to me that you can’t pasken beliefs, I will have to agree to you that there has to be a system to govern our actions and perhaps the system should be that anyone who doesn’t follow the Rambam in this you can’t trust their kashrus. But on that note you should also not trust the kashrus of any rishonim/acharonim who disagreed with the Rambam on these issues.