However, even if true the police are still contributors. They did not restrain him properly in the van and they did not get him medical attention quickly enough. These are even admitted to by the police. Also keep in mind that the other prisoner could not see Gray, he heard him thumping about. Was he properly restrained? How were the conditions of the ride? That GPS thing can be used to determine the route taken. If they left him unrestrained and purposefully have him a "bumpy ride", then it's quite easy to see that he could have essentially been thrown about the van, but not by choice.
So in the end, it doesn't fully answer the questions. Video would answer the questions, but we don't have those.