In the first chapters, it was said that fish contains alot of mana so it might be the reason why the king is drinking the disgusting thing. Some con lured the king into thinking that the juice will extend his life.
You have to remember that every villager there are monsters in their own right and Kerbit is the same. Common sense is just broken in that village that even city destroyers outside would look like puppies in the village.
Tier(angel wife) has been there the whole time. She only intervened because Kerbit is getting full of herself and also to prevent the MC from getting upset and causing(not happening) rampage.
I dont get the butthurt with the name. People being angry about something lost in translation is just pointless as the MC choose a name that would have more meaning to him than just keeping the previous name that he doesnt really understand.