Your supposed slam dunk simply isn't. Take the same example modified twice: "I listened to [Complaining person], a serial killer, and a hapless fool. However,

Your supposed slam dunk simply isn't. Take the same example modified twice: "I listened to [Complaining person], a serial killer, and a hapless fool. However, no one could tell if I was speaking to one person or two." Note that the phrase "a serial killer" could reasonably be read as an aside describing the first party. There is no silver bullet.