Not giving anything away, but more of a cultural question.
Mei is making a speech to the entire student body, however, I do not understand why she addresses to the student body as "Minnasama". I thought one should call peers as "Minnasan", no? As the student council president and also as a senior, she is the most senior member of the student body, so I do not understand why she would use "sama" to any student, let alone the entire student body. The only persons she technically can use that term "sama" is to Yuzu, ie, oneesama, or Himeko, because Himeko comes from has a much much much more respected and old super rich family, ie, a higher social class.
I should be able to answer that! In Japanese, the use of formal/humble language (such as "desu" or the honorific "-sama") isn't solely determined by social status or "rank". It's true that you often use formal language with your elders and those in a higher position than you, but you do so out of an effort to be respectful/formal, not because there's an inherent rule about age or status that you need to follow. So, any situation in which it would be appropriate to be respectful/formal is a situation where it makes sense to use formal/humble language. In this case, Mei is addressing the student body in a very formal capacity, making an official announcement as a representative of the student council. In fact, the language she uses all throughout the speech is dripping with formality, which is something I spent a lot of time trying to accurately represent in my translation. If Mei were to casually address a group of her peers in a normal school setting, I would fully expect her to use "minasan" instead.
I still feel like at the end of the day we are setting up for it to be Mei that's worried about Yuzu and Arata. It's barely there but I got the impression that, that's where we'll go. And now I'm assuming Yuzu will try and hang out with Arata more in order to get to know him.
This is what I'm thinking, too. I've also always felt that, if Arata starts crushing on anyone, it'll be Yuzu rather than Mei, which would be even more reason for Mei to be the jealous one for once.