I would say, "I can't decide which is more pathetic, the way he speaks English or what he actually says."

I'm a TOEFL iBT Speaking Rater and I would have to disagree with the hedge words you suggest in the beginning of thia article (for example, "That's an interesting queston. I've never thought about that before."). In my four and a half years of scoring, I've literally never heard a test-taker respond this way. That would be an appropriate way to buy a little time on the IELTS speaking test, but for TOEFL it would definitely lower a test-taker's score. You only have 45 to 60 seconds to respond to a particular prompt. Wasting time with these kinds of phrases would be judged very harshly by raters who are expecting to the speaker to expand on a topic in a very limited amount of time. 

Can you use both words? Obviously, the scene is important in the sense that you couldn't leave it out and have the play remain the same. In other words, it's not a minor detail ... the scene is essential to the progress of the play, as you correctly pointed out. Relevant means related to the topic at hand; so, for example, tests often ask you about relevant ideas or details. If we discussed the balcony scene, we might say that Romeo's portrayal of Juliet as the sun is relevant to the development of the romance, while Juliet's dress is irrelevant. For a fully nuanced commentary, it seems to me that you could discuss both the importance of this scene and various elements and details that are relevant to the reasons it's important (those which you already mentioned ... romance, personality, plot development).