Revealing mistake: In the shot looking across the four monks in the autopsy room, you can see the one closest to the camera taking a deep breath.
Continuity mistake: Stephanie Watson's supposed killer is, on the TV, named as Mark Kelso, but later when Catherine is going over the DNA sample with Greg, it is a match to a Dwight Kelso - supposedly the same person.
Answer: He doesn't have the authority to arrest him - when anyone is arrested in the show, a police officer does it. Also he tells Ty that he is going to be submitting his case findings to Ty's insurance company, who would no doubt contact the IRS, who then would have him arrested for fraud etc.