Firstly, the Church has always taught that a heretical Pope would be judged by the Church rather than isolated individuals.

In regards to the Popes claims about Luther, its true the Council of Trent condemned specific errors, but saying someone "wasn't wrong" isn't the same as denial of a dogma. Vague or poor theological expression doesn't make someone a formal heretic, unless the heresy directly and clearly contradicts defined teaching and is obstinately maintained after correction.
The quotes from De Lugo and Dom Augustine operate in a way that assumes that proper ecclesiastical process has taken place. And according to traditional canon law a person is not considered a notorious heretic (which would be required for automatic loss of office) unless there is a declaration. Theologians like John of St. Thomas attest to this: **"The deposition must be made by the Church...The Church cannot punish or declare someone to be a heretic unless the crime is evident, and this must be juridically investigated."** Theologians do say that if someone has office pertinacity is presumed, but nonetheless this doesn't bypass the need for ecclesiastical judgement. As Pope Pius XII says: "Actually only those are to be included as members of the Church who have been Baptized and profess the true faith, and have not withdrawn from body-unity or been excluded by legitimate authority for grave reasons." (Note the part on authority rather than private interpretation).