1) To be fair the series frames the cultural issue as if it's a human Vs demon thing which I always found a bit insulting in these types of stories. I get its for simplicity, but human themselves are an incredibly varied species with a lot of different cultures with their own norms, traditions, and beliefs having developed. I mean really this just frames these humans and supposedly all in this world as living in an incredibly small and narrow world without much cultural variety which is just kinda sad.
2) I'm not saying cannibalism is a better answer, but there is more logic there. I wonder what the rationale for burying loved ones in dirt was way back when. Like today we know nutrients go back into the soil (actually we know that and still opt to bury them in a coffin that's never actually seen again and doesn't stop bodies from decomposing and kinda just commercialize grief and the inevitablelity of death...), but like what was the thought before we knew? I'm sure an anthropologist has done an interesting study on it.
3) Good lord. I feel so sorry for the demon lord's army. The good leader can't be bothered to fight for his throne and the betterment of his people the second he thinks they don't want him and the army itself just let's the idiot that took the throne slowly bring their entire country to ruin.