“So it is more a disagreement with how the Westerners tend to think about Original Sin, rather than with what the Catohlic Church in fact teaches.”
Teaches NOW, Alex, teaches NOW. What you have posted from sections 404 & 405 of the Catechism flies in the face, or so it would seem, of Ineffabilis Deus. Be that as it may, its probably time to start teaching this more patristic understanding of Ancestral Sin to Latin Rite Catholics.
I read the entire encyclical and found nothing in it that teaches anything regarding the nature of the Original Sin, let alone anything that would "fly in the face" of its definition in canons 404-405.