“What lone authority even existed in the first few hundred years to make such a decision?”
Didn’t you just assert that the Apostles did exactly that?
So how do explain away Marcion? If there existed an Apostolic Canon was it lost by the mid 2nd century? Is that your claim?
“So how do explain away Marcion?”
The heretic Marcion!? How do you explain away Ignatius who was devoured by lions, who confessed that Jesus is God, the eternal perseverance of the Saints, and many other essential non-heretical doctrines? How about Polycarp, who was burned alive for his faith and wrote to the churches about the scripture they had in their possession? How do you explain away Irenaeus, who spoke of the 4 Gospels and quoted from almost every book in the New Testament?