He's of the opinion that Terrence Mann died right before Costner first met up with him. This is why he was able to go off into the cornfields with the ballplayers, while Costner had to stay behind. I pointed out that his brother-in-law interacted with Mann, so that would be a strike against the idea (sorry about the pun) but his defense was that Costner was sort of a mythical figure who was able to transcend between the world of the living and the dead, which is why he was able to channel the baseball players, which is why when Mann and Costner were together, the brother-in-law could talk to him.

Terrence Mann wasn't dead, at least not when Costner first met him, the best reason being the one that you gave. He was invited to join the baseball players, so at that point you could say he "died" or simply crossed over. I believe that Mann also interacts with a vendor at the baseball game in Boston, so that puts another hole in your brother's theory. Also, if Costner was a conduit between the two worlds, then why couldn't his brother-in-law see and interact with the baseball players, as much as Mann? A better question is, was young Archie Graham a spirit when they picked him up on the side of the road, or was he reincarnated, or something else, before he was invited to play?

Costner had to ask some of the locals where Mann lived. And even though he was a recluse, his death would've been publicized.

I don't think he's dead. There's the scene where Ray reads the article in the paper about Terrance being "missing" and Terrance needs to call his son to let him know he's okay. If the article was about Terrance's body being found deat at his home that would be one thing, and I doubt Terry's a zombie walking around Fenway & Iowa!

Terence was COMING BACK after he visited the "Field of Dreams"...he was going to write a book about his experiences...that's why the ball players wanted him to come with them.