The blood is waiting to be found wherever it is regardless of who did it unless he disposed of it amazingly well.
I think its more plausible that he killed her and moved her than the cops planting a bullet, DNA, key, car, and burned bone fragments. They could probably get me to reasonable doubt on most of it but the burned bone fragments I struggle with. I don't think they hit the blood/hair in the back of the car with as much detail as they should in the documentary. Seems like someone pretty clearly moved her body back there and if Steven's grand plan was to burn it, I'm not sure he knows enough to be concerned about bone fragments.