Being the “head” doesn’t translate into infallibility. For example, my patriarch is a head and he isn’t infallible. And by bishop is a head with many responsibilities, yet he isn’t infallible. The same applies to the Pope. The Pope cannot be the final arbiter of truth if there exists an approved tradition in both East and West stating that Popes may lose the Faith and become heretics. Such a Pope wouldn’t be up to the job of final arbiter, as this role requires an inability to fall into heresy. The tradition of papal fallibility that persisted into the last century even in Catholicism ensures that the patristic quotes you provide praising the faith of Rome and the faith of individual pontiffs don’t translate into the Pope as the final arbiter of truth in the Church. If they do, the ancient Church held to a more developed form of the Papacy than did the High Middle Ages (which believed that popes could err), which isn’t tenable. I would also add that the final arbiter of truth is that faith of the Church which can be traced back to the apostles. This may seem sloppy, but it was advocated by many saints as sufficient, whereas popes could always fall and be judged by this apostolic Faith.