Other mistake: Episode 18 Season 5 "Spark of Life": Late in this episode, the CSI crew is sitting around discussing the case. Sarah Sidle summarizes the facts in the case, including making the comment that Tara Matthews was attacked by the truck driver "just before her death"; in actuality, while Tara was severely burned, she remained alive in the hospital throughout the entire episode.
Other mistake: There is no way a body can be pulled out of a grave the way they showed. It would be a pretty tough trick to pull off with a 6', 200lb piece of lumber and virtually impossible with a body. If it was a solid, non-flexible piece of material, pulling on the rope would drag the item until it hit a ledge and then flip it out of the grave. The body is very flexible, especially at the waist. You'd have to pull a body from above, which they end up doing in real life.
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.