I think you need to give considerable latitude to what might have happened since both of the people were very drunk, almost to the point of unconsciousness. One scenario is he existed the car and was leaning down to vomit (which was all over him) and she hit the back of his head with the spine of the tail light, as he braced himself his right arm ended up in the wheel well and was dragged a short distance backward, and then forward when she completed the turn. You can imagine a hundred other scenarios where the back of his head hit the tail light, the ground, the fire hydrant or whatever and none of them take more than a minute to happen.
The critical things is that there are only 2 ways that you get tail light pieces on the ground and on his shirt with his DNA on the tail light. The simplest is that he and that tail light made violent contact at some point right there in that spot. The second is that Michael Proctor planted all of that evidence without being detected. While MP certainly should not have sent any of those emails to his sister they in no way are evidence that he planted evidence.
During the course of every investigation, police have the opportunity to plant evidence.
If we allowed every defendant to be acquitted simply because it's possible, based on gaps in the investigation, that police planted evidence we might as well close our court system. (and I am in no way naive to think this doesn't happen)
