This is a brief summary of my understanding and conclusions on the Magisterium.
1. Each Pope has the charism of truth and of never failing faith
[dogma: conciliar and OUM]
This teaching is found in Constantinople III, Vatican I, and in dozens of teachings of Popes from the third century to the 20th century.
2. The Pope can never teach or commit heresy. This is not only implied necessarily by his charism of truth (which is contrary to material heresy) and never failing faith (which is contrary to formal heresy) and stated as such by Bishop Gasser in his Relatio to the Vatican I fathers, but is taught explicitly by Constantinople II in the Letter of Agatho, by Vatican I in its doctrine of the unblemished Apostolic See, and by many Popes throughout Church history. [dogma]
3. The Apostolic See is unblemished by any grave error on doctrine or discipline. This is taught by Constantinople III, Vatican I, and many Popes and Saints. The doctrine does not exclude all error, but only grave error. However, grave error is excluded from both doctrine and discipline, whether infallible (no error) or non-infallible (no grave error). [dogma]
4. Only less than grave errors are possible in non-infallible decisions of doctrine and discipline. That which is non-infallible can err, only to a limited extent, and even non-infallible doctrines and disciplines cannot be habitually mistaken, nor can a set of less than grave errors combine to become grave as a set. [theological conclusion, implied by dogma]
5. The definitive teachings of Popes on matters of faith and morals in teachings addressed formally to the whole Church (not general audiences or sermons) are infallible. A teaching under Papal infallibility need not be a definition; it can instead be definitively taught, along with the other criteria for infallibility. Pope have taught infallibly thousands of times. [theological opinion]
6. The same applies to Ecumenical Councils. Every definitive teaching on faith or morals is infallible as the teachings of Councils are per se binding when definitive. This means that the teachings of Vatican II are generally infallible, not generally only non-infallible. [theological opinion]
7. No Pope can be deposed by any person or group, not even by an Ecumenical Council.
the reproach of Christ 
Great summary, thank you for this. On a forum I used to frequent, someone argued in defense of Lefebvre that we cannot judge him for his disobedience to the Pope because, after all, we can’t know his heart, and after all, some Popes ordered people to murder people, so we can’t take it for granted that the order not to ordain was valid. That really didn’t sit right with me and was a major impetus for me to take a hard look at even the “benign” formulation of the traditionalist position. The hypocrisy and mental gymnastics required to pardon disobedience and distrust of sainted Popes while going out of one’s way to defend people who have routinely accused Popes of five heresies before breakfast is simply astonishing. You can’t argue with these people either.
Thank you Ron. I am including your relative in my prayers.
Thanks so much.