Continuity mistake: In the bit where Nick and Warrick decide who is going in the pool, Nick loses and begins to remove his top while still wearing his latex gloves. The camera switches to a different angle and while he is still taking his top off, his gloves have disappeared.
Continuity mistake: When Sara is trying to match up bite plates. In the first two close-up shots of Sara's face, you can see an out-of-focus shot of Greg's sleeve on the right side of the screen, seconds before he actually appears in the scene.
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.