You guys just use the word "Magisterium" in a too general sense. You make no distinctions for the 3 categories: SOLEMN (pope alone defining truths) vs UNIVERSAL (pope with bishops reiterating past truths in a definitive way) vs ORDINARY (all else that's not the first 2 categories...interviews, sermons, speeches, encyclicals, papal bulls, even conciliar docuмents...it all depends on the language used, because this determines the authority of the teaching).
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The first two categories (Solemn, Universal) are infallible; the 3rd category of Ordinary is not. The difference is the level of authority used. The Ordinary Magisterium can never be infallible because it is never definitive. The Ordinary is able to err precisely because it does not use Apostolic authority to command assent and acceptance.
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The Ordinary Magisterium CAN BE ELEVATED to either Solemn or Universal, if the proper language/intent/subject matter is used by the pope (i.e. he speaks definitively, with papal authority, with the intent to bind the faithful, on a matter of faith/morals). If the proper specifications are not followed (per V1) then the magisterium is just ordinary (and consequently fallible and able to err).
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V2 was of the ordinary magisterium because it was not infallible. It didn't bind anyone to anything. The pope and all the churchmen present were not protected from error by infallibility. They have admitted it multiple times. They have admitted that V2 can be questioned, challenged and that it must be "interpreted" in the light of Tradition. Anything which requires interpretation is necessarily not authoritative. Contrast this to dogma, which is clear, concise and meant 1) to be accepted with no interpretation, 2) with full assent, and 3) necessary for salvation. V2 requires none of this, so it's not protected from error. Christ only promised protection to CHURCH TEACHINGS. Something which can be questioned is not a teaching.