I still don't see how you're getting what you're getting from this letter.
The letter to the bishops of Germany from Pope Pius IX is laying out the fact that all the bishops together, i.e. the OUM, CAN teach error.
I'm talking simply about the bishops throughout the world, dispersed (so, not assembled under a council) teaching X in union with the Holy Father. Theologians consider this (with not further qualification or condition) medium of teaching to be infallible, and de fide.
Per Tuas libenter, what Mith wrote is wrong and what Sean has been saying is correct.
Theologians actually do have further qualifications and conditions for teachings to be infallible, these conditions are, per Tuas Libenter, the common and constant consent as being necessary conditions.
If the OUM approve or teach teachings like the NO, we know that infallibility had no part in it because it is new, it is therefore not a constant teaching even if it is a common teaching of the OUM. We know this because this is what pope Pius IX wrote in his letter. He also acknowledged that all the bishops in Germany, the OUM for all intents and purposes if not at least figuratively, certainly could all agree to teach error and actually teach error.
The whole point is that per Tuas libenter, we know the OUM can teach error and we know the qualifications and conditions to know when teachings are fallible or infallible, either way, the jist is that per pope Pius IX, the OUM certainly can teach error.