I can think of 2 possible reasons for removing her clothing; 1) maybe she and the perp had an intimate encounter and 2) maybe the perp thought that the lack of clothes would speed decomposition/consumption by sea life. Sorry to be so graphic.
As to random rapists, I have to go back to the thought I posted a few pages back. In my opinion, a random rapist would not choose as a victim a 27-year-old, 5'10" 160-lb pregnant woman accompanied by a large pit-bull mix dog. Random rapists go for the easy victim; most often, petite, young women or teens. Remember, Charli was accompanied by her dog, Nala. Don't you think Nala would have deterred a stranger? Charli was quite tall - I have a friend her size. I just don't perceive her as a typical victim, particularly when her dog was with her. Whoever killed her knew her AND Nala, IMO.
As to whether the perp acted alone, I truly hope that there was only one person involved in the actual murder. I hate to think there's a second or even third person willing to help somebody end Charli's life. However, I do think the perp abandoned Charli's vehicle knowing that the "wrecking crew" would take care of it for him.