The sand on the shovel was supposed to come from a place where a bonfire had been. When we see the victim buried, it is under a pier on the beach. There is no pier in the opening scene where we see the accident happen. It would also not make sense to build a bonfire under a pier.
A Law Enforcement Agency would not assign crime scene investigation personnel to protect a witness.
When the crime lab runs a DNA comparison to find out who used the crane remote, they get a 96% match with the victim. They determine it is a familial match, and this leads them to the victim's son. Since we only get half of our DNA from each parent, there should have only been a 50% match at best.
When Natalia is checking the DNA on the computer and Kurt Greenfield's name comes up in the system, it says he has brown eyes, when clearly, his eyes are green.