However, I'd like an explanation of why this is the case. I get that, due to the layers, the type-setting applies first.
However, ability-adding/removing applies before power/toughness. Why do Humility and Opalescence both set power and toughness after Humility has removed the abilities from both cards?
Is it that, once you start applying some of a single ability, you apply the entire ability, even if that application happens across multiple layers? If this were so, the interaction only occurs because Opalescence "gets in early" by setting type, which means that it will still finish being applied after Humility removes its ability?