Pat- How about this from Hill:
“It strikes me that perhaps we should keep an agent with President Kennedy’s body – out of respect for both President and Mrs. Kennedy, and in light of the questions that were raised at Parkland Hospital about taking the body back to Washington for the autopsy. This way, if there is ever any doubt about whether Dr. Burkley stayed with the body until the autopsy, or suspicions about tampering, there will be a Secret Service agent who also remained with the casket and can vouch for the integrity of the body. Agent Dick Johnsen is selected for the post because he is an agent who was with President Kennedy from the beginning and is familiar to Mrs. Kennedy, O’Donnell, and Powers.”–Secret Service agent Clint Hill, Five Days in November, page 124
Also in the same book (I can't find the reference now but Horne made a big issue of it), Hill states that the autopsy doctors knew the throat wound was a thru and thru exit wound.