From a common sense point of view, why would "they" plant a near pristine bullet? Surely they would've known this would arouse suspicion? Why not plant a smashed up bullet?
Who said they (whoever "they" are) planted any bullet?
What if the bullet found by Tomlinson was indeed a different one (just like Wright told Josiah Thompson) and completely unrelated to the JFK murder? ...
Nobody in the chain of custody could identify the bullet, now in evidence as CE399, until it got to the FBI lab. As the rifle was also there, it would be easy to shoot some test rounds and "mix up" the bullets.... I'm not saying this happened, but it is within the realm of possibilities. Who knows what went on in the FBI lab during the first 24 hours after the murder....