Okay, my two cents and I will try to say it so it doesn't get pulled:
1. Charli gets called out to help EX. Why did she go? Obviously, she was not afraid of him. He was smart enough not to say he had car trouble, only that he was stuck in a ditch. Was there any evidence of his car that he had been in a ditch? Mud, grass, etc? I'd be looking at the undercarriage. What does the family say about their relationship? They did not seem to like him. Why? Cheater, abusive, irresponsible?
2. Why was the dog let go? Because the person who did this had no animosity towards the dog. Had the dog tried to bite the person who caused Charli to go missing, it is likely the dog would have been injured or killed. Dog is fine. I have the feeling the dog was either let to run before the incident occurred that caused her to go missing, or that the dog was held in another vehicle while the incident occurred and then released. Dogs can go pretty far and I don't think the distance the dog went is necessarily significant unless the person who caused Charli to go missing took the dog down the road and dropped her off.
3. Cell phone last pinged in a "sharky" area. Clothes found nearby as well? Get rid of the clothes because, well, that's why you find all those feet washed ashore. If a fish or shark can't eat them, they leave them alone. Throw the cell phone either in the water or in the weeds.
4. Dump and burn the car in an area known for the disposal of stolen cars. Big way to cover tracks. Burn the car in case there is evidence left in there, evidence of a struggle, blood evidence, etc.
5. Charli was pregnant and not in a relationship with the father. People can be kind of weird and panicked when they are faced with something they are not ready for, such as paternity, especially when they know they are going to have to foot part of the bill for 18 years. I am not saying he did this, but it has happened in the past.
6. EX failed a polygraph. Now that can just be a case of perhaps he is on medication or was in such a distraught state that he could not be "read" well and it turned out to show he was not being truthful.
I think we all know what happened here and looking at a serial killer is just wasting time. I'll eat my words if I am wrong, but most people are killed or go missing at the hands of someone they know.