After watching
the video that Approaching Infinity posted earlier, I think it's certain that Charlie was shot from the front, he was not wearing a vest, and the most likely shooter was indeed that Taylor guy. I was skeptical about an amateur being able to make that shot, but according to the guy (who is a sniper) it is quite doable and it only becomes challenging around 400 ft away. YT just showed me
this other video of a guy making that shot from a similiar distance to prove the point (he says it was 142 ft - I thought in the actual assasination it was 200 ft?) and he is hitting the target multiple times. No bull's eye, but one point he makes is that Taylor was probably going after the body and missed for the neck, with the end result being the same.
As for the mystery of the exit wound, the only explanation I can think of is that the bullet hit bone and deflected it, staying inside.