This story had an interesting plot point, and executed it really well, building a strange and intriguing "mirror" world while also keeping itself believable and enjoyable... until the last installment. It seems chapter 13 lived up to the superstition, and I wasn't surprised to find that the first reply was Mirage going to town on it (and rightfully so). But that's really just one part of the story, and I think it's important to separate it from the rest, which was actually pretty good.
SpaG wasn't perfect, but also wasn't a huge issue. The writing was quite good, and I really didn't have any gripes with the author's style (except for the music cues, which broke my immersion a lot - changing from text to an icon helped a bit in the later chapters). All that said, if there's anything that stood out as being really good, it's the characterizations: this was almost a cast made entirely of OCs, but they all worked well, the protagonist included (physical therapy can really make a character work). Figuring out how each of the mirrors worked, and learning to think about each character differently, was enjoyable, imo. This strategy wasn't foolproof - in canon, personalities tend to match with disabilities for good reason - but I think that's perfectly within the realm of suspension of disbelief.
Perhaps my only issue with the plot as a whole was Saki as the love interest; it felt like not much explanation was given. They run into each other, get food, he buys her a gift, then a few days later they get hammered and undress. For a story with such deep and interesting characters, all of whom the protagonist spends lots of time with (and one of whom helped him tremendously during therapy), it felt dumb that so little time and exposition was put into his love interest.
But on to the story's major failing: the final chapter. Plenty has already been said about it, but I'll go over my own thought process: hospitalization is an important part in many stories, and can be an important part of a character's growth, not to mention a relationship's growth. At the end of chapter 12, I was a bit sad, but also hopeful that the resolution might add some structure to the relationship. Instead, the protagonist decided that he needed to talk to Saki
immediately to fix the situation.
Luckily, in his possession was a device with which he could contact almost anyone in the world with a few button presses, either by voice or text. Unluckily, that skull fracture must've gotten to him, because he immediately forgot that implement - despite using it a few seconds prior - and tried to physically travel the miles between him and his lover. Even more unluckily, the friend of his who was supposed to be watching him, who knew that he relied on crutches, who possibly knew he had a pre-existing spinal injury, and who
definitely knew that he had just suffered cranial trauma which required hospitalization, decided to help him leave the hospital (covertly, because any sensible person would've stopped him), get a taxi, ask the driver to step on it because for some reason time was of the essence, circumvent the school's night guard (who would surely have called for medical attention because they're sensible), and show up unannounced at his lover's door, despite having a serious injury. And she's cool with it. And then the story ends, because cleaning all of that up would've been an
absolute bitch.
But yeah, an interesting story overall, which I think I can say I thoroughly enjoyed, if I focus on the earlier chapters and not the later ones.