Continuity mistake: When Gil and Catherine catch the couple in the bedroom, they are all sitting in the living room. Amanda is drinking a soda and it is about half full. Gil then asks for the straw. When we look at the soda again, it's full.
Plot hole: Amanda Haynes is Lacey Duvall's murderer, however in the opening scene she was kissing Patrick Haynes at the party downstairs at the exact time Lacey fell into the pool. She could not have pushed Lacey upstairs. (00:01:16 - 00:41:29)
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.