It complicates the game to stipulate that Monty knows where everything is and doesn't want you to win.

That's one way of playing.

But in the version discussed by me, he doesn't know and he doesn't care if you win or not. And there might be one or two clunkers. I could not understand how, in that situation, switching would increase your odds.

And it now looks as if it wouldn't. But if Monty knows and you know he knows, that's a different story.