Which isn't practical for multiplayer action games, so we end up here.

To do real time analysis and interception probably not. But for after the fact analysis, if a player is moving on knowledge he couldn’t have had because it shouldn’t have been rendered yet or something, then you can assume cheating.

This. Also the client knows more than its allowed to show the user, like the positions of enemy players. You can make aimbots and wallhacks without needing to tamper with the game state.

Doesn’t matter. There’s no world where a multiplayer action game is worth it, and anyway this is a classic example of trying to solve a social problem with technology.

The reason cheating is a problem at all is that instead of playing with friends, you use online matchmaking to play with equally alienated online strangers. This causes issues well in excess of cheating, including paranoia over cheating.