The reason I picked out the teaser is because it has often cropped up that the Holy Spirit will prevent the Pope from saying or doing anything contrary to the faith.
The opinion that a Pope never will turn a heretic is the first of the five opinions Bellarmine presents. It is by Albert Pighius, a 16th century Dutch theologian, mathematician, and astronomer.
The work of Journet quoted by Wathen,
The Church of the Word Incarnate, was written in 1955, and doesn't seem to be available online, except excerpts, which appear to be the parts pertinent to the "heretical pope" topic. Here a link for reference:
An extract from The Church of the Word Incarnate