I was just looking more closely at your post now that I had a little more time this evening, and want to mention the テンション translation on Pg. 16. This word is a false friend. It is borrowed from English (tension), but from everything I have read, when it’s being used in Japanese in conjunction with 高い, as it is here, a better translation would be closer to “energetic” or “hyper.”
At first blush, I translated it like you did, “Why is it so tense in here?” But then I looked again and realised it didn’t quite fit with the sentences immediately around it (it felt like a jump; why should Sanae-chan find the question of tension in the room annoying when it doesn’t affect her, right? If anything, you would think the same curiosity would be there), and that was when I remembered that テンション doesn’t mean exactly what it sounds like.
A better translation of that phrase, I think, would be, “Why/how (なんで can mean either depending on context, and both work equally well in this situation) are you (meaning Mina) so energetic in the morning?”
I could be wrong, but I’m fairly confident in that.
Anyway… I should quit procrastinating my reviews, so I’m off!