1) Why is it important to apply PACC when communicating in different social situations?
PACC helps you to identify suitable tones to use when speaking as you can extract the purpose of the conversation and identify the type of audience you are communicating with. It also helps you to identify when you should speak politely (formal) with a person or speak like a casual chat (informal).
2) If a majority of your classmates hold a certain view about a controversial topic, while you hold an opposing view, how would you convey your thoughts politely?
Since the purpose is to let my classmates, who are the audience, know about my thoughts about the controversial topic, I can choose to speak informally but also with a polite tone to let them know about what I feel about the topic. I can start with "From my perspective, I feel that....."
3) What are the possible repercussions if the tone and register were used wrongly?
If the tone and register were used wrongly, for example, when a supposedly formal speech turns out in informal language, the audience that will be listening to the speech would feel that this matter is not taken into account by the speaker and may think that the speaker is not serious about the matter.
Another example is when you talk to a friend using formal language. If it happened to me, I would feel very weird as that was supposed to be a casual conversation and the person should be using a light and casual tone.