Continuity mistake: While Kelly is being interrogated in the interview room by Nick, her hair changes between shots. Close-up it hangs loose. In the wide shots both sides are tucked behind her ears.
Audio problem: Warrick is examining the garage for evidence and sees a stack of crates of soda, he sees one has a bullet hole in it. He picks it up and shakes it, it rattles, demonstrating the bullet is still in the can. He pops the ring to open it and it hisses, despite the fact that the bullet hole in the side would have let the pressure out. They clearly used an unopened can to film 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.