Before we can answer this question, when was Peter given the keys of the Kingdom? After all, this is your doctrine to defend, not mine to undermine. There is nothing in Scripture that says he had exclusive authority, and clearly ancient Tradition doesn't back it either. He didn't get them in Matthew 16:19 because Jesus is talking in the future tense: "I will give you the keys of the kingdom of heaven". It has nothing to do with Isaiah 22, because the key of David was given to Jesus, not Peter: "And to the angel of the church in Philadelphia write: ‘The words of the holy one, the true one, who has the key of David, who opens and no one will shut, who shuts and no one opens." Revelation 3:7. And, every other time Peter was given any authority, it was generally given to the Apostles as a group. Don't forget, Jesus saying he will give Peter something doesn't mean he is giving it to Peter exclusively. So, what's your argument?
who is the true messianic king, not merely the prime minister or the Messiah’s chief representative...Jesus or Peter?
Jesus is the messianic king; Isaiah 22 is talking about this messianic king; therefore, it's not relevant to Peter having the keys of the kingdom.
Let me be blunt, the Pope isn't the prime minister or the Messiah’s chief representative on earth unless Peter was given the keys exclusively and those keys were exclusively passed down through the popes. Therefore, it is circular to argue for the keys being exclusively given to Peter because he is the prime minister or Messiah's chief representative on earth.