You've seen the second male lead losing
You've seen the second female lead losing
You've seen the childhood friend losing
Now get ready for
The supporting character losing
Seriously though give my boy Earth this one he deserves happiness
Why would someone leave a karaoke early then go back immediately?
From my experience with planning events and observing event planning, oftentimes people will show up for just one thing. It's like how before the "main" comedian for a comedy show they have a few people come in first to warm up the crowd, maybe get them buying a few more drinks; if they did it in the reverse order everyone would just leave once they're seen the person they wanted to see.
In this case, Oura is the "headliner", or "main event" or etc. and in retrospect I should have used like, say a twitch streamer that is inviting fans on stream to play a game. Fans ain't playing Runescape for the sake of just liking Runescape; they are playing it to play with their favorite streamer, and when their favorite streamer logs out, they're going to log out.
In the same sense, they started school what like...a week ago or something? This group barely knows one another and seems to be a mishmash of archetypes you normally wouldn't see together in these types of manga (very refreshing to see no class hierarchy "Protagonist is a loner" type stuff). The connecting glue is Oura, and without Oura it's just a bunch of people who don't known one another very well. A bit of an elaborate explanation but hope this provides some sort of insight.