Joseph Ratzinger's hermeneutic of continuity, which defined his pontificate, is strong evidence that he had no intention of breaking with the Church's teachings that must be believed with Divine and Catholic Faith. Therefore, pertinacity, which constitutes the form of the sin of heresy, is difficult to demonstrate with moral certitude.
Ratzinger/Benedict XVI was a pre-conciliar theologian. This means that the did serious studies. I think that it is highly creative to imagine that he did not know that he was proposing and teaching things that were contrary to the faith.
He was highly intelligent and qualified. He knew very well what he was doing.
When we read the Moral Theology manuals, we see that the requirements for mortal sin are not so high. You don't have to have a deep moral knowledge to commit a mortal sin. Yet, when some people talk about conciliar Popes and heresy, the bar is set so high that it is almost possible to say that Luther was not a heretic.
Possibly excluding John XXIII, all of the conciliar Popes heard repeated warnings that they were going against the Faith, yet, they all continued on their wicked ways. Can they be excused? I honestly don't see how it is possible.