Ideally player A shouldn't be networked player B if there is a wall between them but what happens when they're at the edge of the wall? You don't want them to pop in so you need some tolerance. But having that tolerance would also allow cheaters to see players through walls near edges. Or your game design might require you to hear sounds on the other side of the wall (footsteps, gunshots, etc.) which allows cheats to infer what what may be behind the wall better than a person would.
Yes, and you cannot prevent this except in in-person tournament.
Any output send toward the player, even a faint audio queue could be analyzed, and use to trigger an action or display an overlay to the screen, and no amount of kernel-level stuff will prevent that, as you can do this outside of the computer running the game.
You will own nothing and like it.
You can even do this calculation on community-run private server.
But as you are pointing out, forcing client-side intrusive anti-cheat is cheaper, thus this as nothing to do about preventing cheating, but about reducing cost.