We are Catholic and different from Prots because we hold to the truth which comes from Christ, which includes the Magisterium, but which the magisterium (the teachers in the seats) can err from.
No, you completely ignore the formal motive of faith and rule of faith. Someone could hypothetically believe all the Church dogmas, but if they discerned these based on their own private interpretation of Scripture, then they do not believe them based on the proper formal motive of faith. I say hypothetically because in the practical order it's impossible for private interpretation to lead to all of the exact dogmas taught by the Church, which is why there are approximately 23,000 different Christian "denominations" out there.
Indeed, not every pronouncement of the Magisterium is guaranteed to be free from error, but we're not quibbling about the notes or extent of infallibility here.
With the Conciliar Church we have an institution that has become so corrupted that it lacks the marks of the Church, that the Magisterium is thoroughly polluted with Modernism, and is leading souls into grave error. We have a Universal Discipline that produced and promulgated a Rite of Worship that is offensive to God. We have the canonization of the chief culprits in this destruction of the Church. All this, when, taken as a whole, would constitute a defection of the Church. Or, to put it a different way, when the Church has officially (in its Magisterium, Worship, Canon Law, etc.) gotten so bad and corrupt that it requires that we separate from submission to this teaching and this form of worship, i.e., that it requires the existence of a Traditional movement, then that would constitute a defection of the Church. If that doesn't, then there's no such thing as a defection of the Church.
We're not talking about a problematic statement or two in Vatican II. If that's all there was to this, there would in fact be no Traditional movement. At that point, it would be a situation of respectfully questioning these through the appropriate channels. See the thread I started regarding all the statements of +Lefebvre declaring that the Conciliar Church lacks the marks of the Church.