So I'll start by saying I didn't read the oneshot so I've no idea what is happening there even if with all the comment I saw I can guess it's bad news.
So I'm wondering if Saeko's hair aren't smh important. Like at the start when they met her hair colour was completely done and more we advance more her hair start to turn black again (which is normal if she doesn't make her colour again). And in the first few pages when we saw them already together her hair were yet blacker meaning it's clearly after what we just saw (which was clear anyway but that just mean the author is careful about this). So that's why I'm wondering if there more behind it. Like how black are her hair when shit start to happen in the oneshot? It could be some sort of clock telling us when all it will happen or something and that it will all happen when her hair as as dark or something. Like to help us with the chronology. Or even a hint that it won't happen like it happened in the oneshot if her hair were darker in the first pages of chapter 1 than when things went bad in the oneshot. But I guess only someone who read the oneshot could tell.
Anyway I think I've gotten paranoid because of all the comments about the oneshot's end smh.