While this is not the greatest way to introduce a "love rival" (and really, it's more to emphasize Mashiro's insecurities and give her character growth in that general area than a real love rival IMO), I don't think it's as contrived as people are making it out to be.

1) She's a junior that's clearly enamoured with Nagisa
2) Wanted to get into the same school
3) Would have gotten into the same school if extraneous circumstances didn't happen (and if you're getting into prestigious high school, chances are you don't have the freedom to downgrade to a third-rate one without said extraneous circumstance ["Hey mom/dad/teacher/advisor, you know how I'm getting into that great high school? I'm going to the ghetto school instead cuz I'm chasing after my crush" doesn't fly unless you're a main character])
4) Sees another girl in Nagisa's arms
5) Loses it, does some inane rival challenge thing like these characters are wont to do

All of this is pretty standard, it's just the random encounter in the middle of the street that's the issue. Switch it up so they're either walking by the old middle school, or the prestigious high school, and the encounter is suddenly less contrived since it's a place where this type of encounter might happen.

When the trope is so overblown it memes itself into replacing reality and a more realistic portrayal has all the critics crawling out of the woodwork to complain it's not overblown enough.

"a grand ocean-liner of love!"
"So something that will sink"
best part right there.

I enjoyed Yuzu and almost everything around her, if that's all the manga was it would be stellar. Mei, and almost everything revolving around her, drag it all down into the mud. I don't blame the author for the rushed ending, it was better to put the whole thing out of its misery rather than try and salvage the wreck.

I feel like this could have been better if it was something along the lines of "MC gets dragged into (local-level) team-based fighting game tourney because [reasons], has to learn to play fighting games from no experience to help [friend/classmate/whomever] win". You have your training/gay during break periods, and during the tourney you could have a bunch of fighting game action panels and all that. I mean, it's not a great idea, incredibly cliched and whatnot, but to me it'd be better than what we got.

instant death is great, it requires you to believe in the heart of the c- dice, and gamble your fortunes on luck.

I feel like that self-destruct info bit is foreshadowing; I can fully see Granite (that's the mage's name, right?) learning self-destruct, not realizing it kills you forever, and some tragic drama when she uses it, or attempts to, to kill Goblin.

wild speculation in a situation of desperation for justification of a prevarication for fornication is causation for damnation.

Or, maybe she wanted to be her lover in the future, couldn't do so because [reasons], and so she went back to the past to try and change the future so they end up together. That bit at the end of this chapter definitely suggests something like that is possible.

I dunno, I read it as the emotionally damaged bonding by an innocent girl inflicted with the trauma of being put in a battle royale situation, and I feel like I got more out of it than those of you reading it like a battle royale. The ending was still pretty bad, though.