I'm still in the Fredo boat myself. As TB stated, Roth wouldn't have attempted to get this done with the help of a simple housekeeper, someone that he doesn't even know. He would have no way of knowing how loyal that person would be to the Corleones, which could have possibly led to a major backfire of the whole plot. Rocco proved his intense loyalty to Michael later on, so I don't think it could have been him. And Neri is absolutely out of the question. Even the book makes that clear.