I think you summed it up quite nicely. In short, he needs to be a good team defender and everything that incorporates. Tim Duncan, who isn't a great leaper or an freak athlete, was third in the league in blocked shots last season, if memory serves. How does he manage to block so many shots without being a jumping jack? Position in the post and technique. Someone was debating how good a defender Parker was, and decided that he's a poor man on man defender, but is a good team defender. Not sure I understand how one can be critical of a player for playing good team defense, especially when your team is winning.
Cousins can be a good defender is he gets back on defense, learns the proper rotations, and executes them, and makes better decisions. The majority of his fouls come from making bad decisions, either out of frustration, or laziness. You can't stop every shot, but you can contest every shot. One thing is for sure, I'd rather shoot an uncontested shot than a contested one. If the opposing player comes into the game shooting 51% from the floor and you make him shoot 48%, you've done your job. If the opposing team comes in shooting 45% from the floor and you as a team make them shoot 42%, you have a good chance of winning that game. I'm not asking Cousins to be great! I'll settle for consistent with good effort, and let the chips fall where they may.
I hope Cousins remembers one thing Malone said to him. If you want to be the leader of this team, that means you have to be the example of playing the game the right way, not just once in a while when you feel like it, but every game, night after night. That takes focus and dedication. Its hard work, just go ask Kobe or Lebron. But its worth it, and the only way to find out that its worth it, is to do it.