One of the things that we understand from Scripture and the examples of the Apostles is that fallible men can follow infallible God.
Another thing that we understand from Scripture is that fallible men can corrupt the Faith and fall away from an infallible God, so that argument isn't definitive support for any kind of 'Magisterium'.
So again, we are back to the underlying assumption that fallible men who have 'departed from the Faith' and are still part of 'the ultimate authority of the Magisterium' are either ultimately discovered or inconsequential to 'the Faith'.
Any news on how Scripture can only be authoritative where it assigns ultimate authority to 'the Magisterium' who then serve as the 'ultimate authority' for interpreting the rest of Scripture?