you can go over the rules time and again (not to pick on Buddy) but Buddy is going to miss a clue and do the wrong thing. And at the NBA level it’s harder because guys will do things to disguise their intentions.
I feel a good coach can do some things to manage these limitations. For example, Dave said once he didn't like Belly taking more than three dribbles. If Luke applied this constraint to Buddy last year we wouldn't have seen as many bonehead passes. But yes there is a limit.
As for players like MBIII, I believe his defensive limitations are due to his coaching. Doug, as well as some posters here, have commented on his technique being poor. This scouting report says his brothers defence is also clueless. But that might not matter, because if he can't stay on the court he is going to find it hard to make substantial improvement.
Taking it back to the larger argument - what is needed to create a playoff team - a simple coaching change or a complete personnel rebuild? I sit in the rainbow group that thinks the core roster we have is good enough for the 7/8 seed. But think if that was the goal at the start of the season we should've kept Len and Baze. I wouldn't reject player change. Buddy would be great as a microwave off the bench but he doesn't want that role with us. HB is a model professional that can get you a bucket when you need but he's too nice to be your second option. I want MBIII to be successful here and Sac to get value out of him, but if he's hurt and unhappy, it's going to be hard for him to get better. But selling these guys cheap isn't a solution. And getting new talent for the coach we have would be like buying a new car for someone who doesn't have a licence.