It's one of those damned if they did, damned if they didn't things. Either they planted the key and bullet (in particular) and thus the whole case is undercut, or they were extraordinarily incompetent in their initial searches to have missed them that they were discovered days after initial searches, and that kind of undercuts the whole case too.
I've still never heard a good explanation why he was putting her in the car in the first place if he burned her feet from where he allegedly killed her (or why he randomly had a few other locations to burn her). I don't understand why someone so careless as to park her vehicle on his own lot, with her and his blood in it, is suddenly such a whiz at covering up a crime scene that no blood is in the house or garage. Speaking of which, they talk about blood swipes in her care that represent her hair sweeping across. Did they show that was indeed her blood, and how do we think that got there if we are saying his blood was planted...did they plant some of hers in her own car too, or was that part of the actual crime scene?