"Then I point out that Sola Scriptura is really impossible. You're always taking someone's authority on what the scriptures say."
Exactly - anyone could write a book and claim it to be the Word of God, and proclaim it to be true. If you or I did that it would surely be questioned. There must be an authenticating source. So, in a sense, all protestants who claim the Bible is their sole rule of faith are accepting the claim of authority of the Catholic Church, since it was that Church that decided the Canon.
Yes. In fact, John Donne pointed out to the congregation in one of his sermons preached at Westminster that the first Christians had to ask the Jews which books of the Bible were canonical.
Not until later was it decided which books of the New Testament were canonical, and of course there is still some disagreement about the status of the Apocrypha.