Do I really need to use an Oxford comma?
The distinction: "My heroes are my parents, Wonder Woman, and Jesus" vs. "My heroes are my parents, Wonder Woman and Jesus." If you wish to be specific, use the Oxford comma. If you wish to imply that your parents are a fictional superhero and the Son of God, leave it out.
How can I recognize a modulation as opposed to a moment of tonicization?
The key (pun not intended) is to find a cadence. A strong cadence and extended length of time spent in the new key indicate modulation, while tonicizations are passing.
In what cases does the "passé simple" come into use? Quand est-ce qu'on utilise le passé simple?
Generally speaking, one sees the "passé simple" in writing. In speech, the "passé composé" is preferred. However, it is still important to be able to use and understand the "passé simple," as it is consistently found in French literature. Le passé simple se trouve dans l'écriture littéraire; lorsqu'on parle, on préfère le passé composé. Tout de même, c'est important de reconnaître et de comprendre le passé simple, car on le retrouve habituellement dans la littérature française.