If you want, you can talk a bit about what the product is trying to achieve (e.g. what’s the goal of reactions/comments) and perhaps even have hypothesizes for why metric X might be down and metric Y up.
However, I recommend jumping in to the question quickly — for tradeoff questions in particular, most people really dance around their answer and require a lot of nudging/hints to get to a semi-sensible answer.
Being crisp on this question type will earn you a lot of brownie points from the interviewer. You can certainly add more to your response once you’ve answered the tradeoff, e.g. in the “optimization” section, you can form hypothesis about why metric X is down and Y is up and come up with possible product solutions for fixing it, and so on.
Re: timing, goal setting is generally the meatier part of the interview, often half of the interview time or more, including a back-and-forth discussion on the pro/cons of various goals and probing at your thinking. The other two questions are generally shorter and often you’ll only be asked either of them rather than both.