Only time will tell if Rondo ever develops a reliable jumper. That said, I think it's not so much about overthinking, but rather, about consistent mechanics. There's a reason great athletes are often known to have obsessive-compulsive workout (and personal) habits. Repetition breeds consistency. Consistent mechanics yield consistent results. If Rondo knocks 'em down in practice, he needs to find a way to get his mind and body into that space during the game.
On a related note, Tommy mentioned something I found interesting the other night. I can't remember which game, but it was a home game within the last few weeks. He disagreed that Rondo's lack of a jumpshot was major issue. His argument was that although Rondo passes up open jumpers, he's instead breaking down the defense and creating high-percentage shots for his teammates or for himself (i.e., layup/floater). Granted, Tommy's hardly an objective observer of the Celtics...and I don't think I entirely agree...but it got me thinking...