At the end of the day, it is only God's Providence that prevents the Church from being befuddled and even materially mistaken regarding the identity of the Pope. There was the one pre-Vatican II theologian, writing about the so-called "Western" schism, who wrote that we have to be careful not to presume on what God would allow to befall the Church. We know that He will provide a solution, but short of that, he took that crisis as a lesson. He even said that there's nothing to prevent God from allowing a vacancy to go on for a very long time. Before it happened, I'm sure that many Catholics would swear that something like that Schism could never happen.
Even with the Siri scenario, God allowed details of the conspiracy to leak out. And God made sure that the V2 orientations were so over-the-top that even simple Catholics with little education could simply apply their Catholic sensibilities to discern, "uhm, I don't recognize whatever this is as being consistent with my faith." Had God allowed this to go on for centuries, with little subtle changes every decade or so, everyone could have been fooled.