The CE article speaks in your terms, but doesn't address the issue of any Magisterial teaching being incapable of error, which, again, is the focus from my end.
I've never held that the Magisterium is ABSOLUTELY incapable of error, just that it can't be in serious or substantial error. Sometimes people misinterpret these quotes from various popes about the Magisterium being inerrant as referring to an absolute inerrancy, and thus the only way Stubborn can make sense of this is to turn it into a tautology, where if it's true, it's Magisterium, but if it's not, then it's not Magisterium ... which renders that teaching rather moot and almost absurd.
There can in theory be some error in the Magisterium
per accidens, in a manner that does not compromise the overall inerrancy or integrity of the Magisterium ... according to those citations that I have made from Msgr. Fenton.
So, with regard to the greater indefectibility of the Church, what degree of error would entail a substantial change, a transformation of the Magisterium from a generally reliable guide to the faith into one that's actually inimical to the faith?
To me the litmus test is presented to us in practice. If the Conciliar Church is so alien to our sense of Catholic Church that we feel that we must separate from it, reject its teaching and its overall orientation, its public worship, etc. ... that is the point at which this has crossed the line from a myopic discussion regarding the precise limits of infallibility in the strict sense (as it was strictly defined at Vatican I).
We have a much bigger problem here than just an erroneous teaching here or there in a Papal Encyclical or Allocution. We have an entire institution that we no longer "identify with" as Catholics. I've used this thought experiment before, where we would imagine St. Pius X time-warping to our day and beholding Bergoglio, the NOM, and all the nonsense of the Conciliar Church ... right down to the aberrant Novus Ordo celebrations and the half-naked male gymnasts performing at a Vatican audience hall. If you didn't tell him that "This is the Catholic Church." ... would he even recognize it? He absolutely would not. He would think it's some Protestant sect. In fact, Luther would probably not even recognize it as Protestant. If you then told St. Pius X, "This is the Church." he would undoubtedly suffer a stroke and drop dead on the spot. This Conciliar Church, as Archbishop Lefebvre has stated in public numerous times, lacks the Marks of the Holy Cathlic Church, the marks meaning the essential identifying characteristics.
If it were possible for the Papal Magisterium to undermine and harm the faith and establish corrupt moral standards, a noxious and sacrilegeous public worship that offends God and harms souls, this would render Our Lord's promises regarding the papacy to be meaningless. It would render the Church meaningless. When we speak about the Church's public worship, what we mean by that isn't just that it's done in public. What is meant by that is that the Public Worship fo the Church (the Liturgy in Eastern terminology) is in fact the Church herself praying. Can the Church pray to God in an offensive and even blasphemous manner?