You are assuming that those Councils were infallible because of their content which is wrong. They are infallible because they are an act of the Magisterium.
V1 would disagree with you.
The Church decides what is contained in the Deposit of Faith. Those teachings on faith or morals are our Rule of Faith.
Your previous post is also incorrect, and both are interconnectedly wrong.
The Church decides what is contained in the Deposit of Faith - yes and no.
Yes - She does when there is controversy and She needs to re-affirm the clear teachings of the Faith.
No - She does not have the power to issue NEW articles of faith, only to re-affirm, re-teach and clarify.
The deposit of Faith was already handed down IN FULL by Christ to the Apostles. It's sorta like a deck of cards. Christ gave the Apostles the ENTIRE deck and each card represents an article of faith. Each article of faith is what catholics learn and memorize to become catholic. When a heretic or pagan comes along and says 'well, there's also a queen of rubies, in addition to the queen of diamonds', the Church says: "no, only 4 queens exist". If someone asks, "well, what about the joker? Is he part of the 52 cards?" Answer: No.
Point is, the Church did not create the deck of cards, just as She did not create doctrine or the articles of faith - Christ did. The Church did not create herself - Christ did. So, the magisterium IS NOT the rule of faith, though she can make rulings. She is the GUARDIAN of the faith, and the faith is composed of doctrine,
ergo doctrine is the true rule of faith, because it existed BEFORE the Church, because Christ existed before the Church. She cannot create, change or delete doctrine, therefore She cannot be "the rule" of our faith.
Doctrine = rule of faith, because ALL doctrine existed before the Church.
Church/magisterium = teacher of the faith, who can issue "rulings" when necessary.