> What does the protagonist brother-in-law do to make enough money to own a castle and golf course?

It was family land passed down for generations. It is a clearly a manor. The staff were sent home after the wedding for the end of the world.

Everyone knows about the "death dance" and he is just a disbeliever that is trying to keep his chronically anxious and depressed wife and sister-in-law from being even worse due by ignoring it/being skeptical. He is obviously a liar, though, and believes it is coming.