My argument is based on papal teaching (often repeated) that the Magisterium is free from error.
Let me re-phrase this whole question.
1. The Magisterium is the teaching office of the Church.
2. Only the Pope can teach INFALLIBLY to the Church (whether solemnly or non-solemnly).
3. The Magisterium, outside of the pope, cannot teach INFALLIBLY.
4. The Magisterium, outside of the pope, can only re-teach "that which has always been taught" since they do not have the power to clarify or define truths 'with certainty of faith'.
5. Only the pope's magisterium can declare teachings that have 'certainty of faith'.
6. If the pope is not infallible and indefectible in all things (and he's not), then the neither is the Magisterium.
7. The magisterium is only infallible/indefectible when SOLEMNLY declaring a matter 'with certainty of faith' or that it is 'consistent with Tradition'.
Ergo, your belief that the magisterium is 'free from error' only applies if you are talking about the 2 situations in Pt #7. Outside of these situations, the Magisterium CANNOT be infallible/indefectible because IT IS NOT ABLE TO CREATE NEW DOCTRINES. So, if a teaching is new or novel (like in V2) it is not "free from error" because that's impossible. If the Magisterium is to be free from error, outside of infallibility, IT HAS TO PROVE THAT THE TEACHING IS CONSISTENT WITH TRADITION. If it cannot prove this, then IT'S NOT AN APOSTOLIC TRUTH, and thus, IS NOT CATHOLIC.