If comprehension is the focus and goal of all literacy instruction, how can abstract skills, like the difference between strong and weak vowels in Spanish, be taught within a context that emphasizes comprehension?

Students learn discrete, abstract skills (like weak and strong vowels, or the difference between the hard and soft c) best by using words, sentences, and paragraphs they already understand. For that reason, the words used to teach these skills come from words that are tied to a unit of instruction or other highly comprehensible context, like their own names. You can see the comprehensible instruction of weak and strong vowels in this video series: Nombres cortos y largos.