I know Kuga is Manipulative Bitch, but how the hell did she manage to traumatize people with Irises like "see what's for dinner"? It doesn't seem to be even close to a level of trauma one may get from seeing others' lies or even being able to guess "does this person is suited to be may boyfriend". It's hard to believe that kind of power to be a curse.
It's pretty simple - because of her past, and her iris, she got quite good at manipulating others' feelings over time. That means she doesn't need to convince them that using their irises is bad, she has to instead crash their minds to make them avert using it, either consciously or subconsciously. Which is easier than it sounds, considering that they're teenagers.
She also doesn't get everyone with the curse, merely that the ongoing Iris hunting is crushing the student body in general. Everyone's afraid they'll be next, and possibly that the curse might be infectious (note how Toru is treated, as if mere proximity can spread his "Zero" status). It's amazing how you can oppress a whole group by making an example of just a few prominent members. And if it was an Iris power (as most would immediately assume), then by the logic of Irises, it must be something that requires looking at someone, perhaps even meeting their gaze. So nobody wants to look at anyone else (how do you know the other person isn't the hunter? how do you know the victims can't spread it?) and any Irises that are based on reading other people become effectively negated even without the hunter's direct intervention.
Did Asahi feel anything concerning what she did to Toru? I'm sorry, but the severe lack of an apology from her just makes their reconciliation confusing to me. Why would, should, or how could he forgive her for ruining his life and how can she have such a severe lack of guilt about it? Their entire relationships as teens makes absolutely no sense to me.
Probably the big deciding factor is Sasamori. Toru couldn't really bring himself to say no to her even from the start - being around her lets him forget, if only for a moment, how much he's hated and how hard he had to work to attain a modicum of peace in his life. He can "forgive" Asahi for Sasamori's sake, and pack his resentment away enough even to fool himself and her Iris. And yet they really don't socialize at all unless someone else (usually Sasamori) is present, or else Asahi has a problem she wants Toru to solve. They're not really friends, which Asahi may be hinting at when she brings up all the tensions within the group. And can you imagine Toru actually accepting an apology if she came to him with one? More likely he'd deal with it like he deals with everything else: shut up, pretend it never happened, and walk away. It's the kind of "forgiveness" that comes from a tacit agreement not to talk about a problem anymore, without solving anything.
While you can argue that Asahi wasn't quite as guilty as she should been, it's not quite as clear cut. As much as I feel sorry for Toru, he did rat out his childhood friend, even if it was under pressure. Asahi was reckless, and her brutal honesty and sense for justice definitely costs her in her focus chapter. Asahi learns to accept Toru, and chapters have shown they've basically reconciled(defending him from Houjou, confronting him on Sasmori's feelings). Since Toru is the protagonist, it's more likely that he has to take the high road rather than trying to get revenge on Asashi. They were also reckless children, so maturation from both sides is needed.
Ratted out or not, that does not justify condemning him to a life of being ostracized and continually bullied. As far as I'm concerned, she's lucky that he didn't go insane and become an Iris serial killer, with her being Numero Uno on the list.
Which was not deliberate on her part - let your mind process that. Yes, she did cause him to suffer by exposing the fact that he was an "Iris Zero". But she didn't do it out of spite - she was merely a child who was unaware of the possible consequences of her actions. From the very beginning she did not like it when people lied and was capable of extreme reactions when that happened (she hit a teacher for that, which in Japan is Serious Business), and yet she didn't do anything when he lied about having an Iris because they were friends. She only revealed his lack of Iris because he revealed her, and she did it in a "you told my mom I ate a cookie so I'll tell your mom you also ate a cookie" way because she trusted him as a friend not to reveal it to others. What she did then was childish, immature, petty, stupid, and devastating. But it was not deliberate in its consequences - she did not want to cause her friend to suffer like that but merely wanted to reveal something he tried to hide because he revealed something she tried to hide (a fairly normal behaviour in children), and had she known about the consequences back then she likely would have just hit him and left it at that. In fact, considering her behaviour in the earlier chapters it is very likely that she had wanted to apologise to him after seeing how others started treating him. But it's one of these situations when just saying "sorry" won't do it, and as her guilt did not disappear over time she still did not know how to apologise properly, always thinking that "it won't be enough", expecting him to be as petty as her own actions were and thus not to accept her apology, which would bring him from "a friend to whom I caused pain and should apologise but don't know how to" (which is why they still call each other by their names instead of surnames, something which in Japan only happens to people who are fairly close) to "someone who resents me and doesn't want to have anything to do with me" because she expects that he would blame her for all of his suffering and be bitter about this, which in itself is a misconception of hers caused by her guilt. You can see her fear of it happening when he tells her that he won't help her - for someone whose expression is always either neutral, happy, or angry, that was quite a guilt-laden grimace. Her other actions support that as well - her being surprised when her he helped Koyuki as that could cause him to suffer more and she of all people did not want that, her playful comment about "not being friends with someone like him" after he showed her that he is mature enough to forgive her regardless, and her further actions when she was a friend to him, trying to make him feel comfortable. So saying that "it does not justify" means that you expect a child to act with the maturity, thoughtfulness, and social awareness of an adult. And I am not trying to say that there aren't children capable of that, but they are rare exceptions and expecting such capability from some random child, especially one with issues of the sort she had, marks one as someone who is either stupid or emotionally dry. And if anything, we saw that Toru is capable of analysing people. Therefore, he likely managed to see how she felt about her "betrayal" of him and thus she needn't apologise if she can't.
It's important to note that even before their reconciliation, Asahi does how more interest in Toru than other members of the Student Council. While they're all beginning to laugh at him, she's still visibly shocked by his presence, and remains so while the other students begin gushing about how Koyuki would make a great Council President. When he leaves, she also stands up and says his first name, and appears to be moving as though to follow, but is beaten to the punch by Koyuki. Then, the last two pages of the chapter is Asaki watching Toru and Koyuki talk a few days, with the very final panel showing Asaki saying Toru's full name in almost longing manner. When you take in consideration the fact that they were apparently good friends as children, it's not much of a stretch to conclude that part of Asaki did feel guilt of being one of the root causes for Toru's bullying - and, possibly, such guilt could be one of the causes of her own "strong sense of justice" and desire to become a police officer. As for why Toru would be so forgiving, I'd chalk that up to the fact that after being an outcast for so long, he's not about to turn away an old friend who's never been shown to be cruel unprovoked, especially when said old friend is also friends of his two newest (and indeed, only) friends.
How many people with Iris is there? I've read three different stories that tell us roughly how many are born with it. The one here that says it went from being 1% of born babies until five years ago, one that says that five years ago the rate babies born hit 99(.997)% about five years ago but the percentage was steadily growing before that, and the one on the frontpage that says that they popped up 27 years ago and skyrocketed to 99(.997)% five years later. The first one makes no sense, the second would have roughly half of all people Toru's age be Zeros, the last make the most sense in the context of the sheer amount of Irises that run about, but something about it doesn't sit right.
Mistranslation. Rather than “until 5 years ago”, it was supposed to be “over the course of 5 years”. It means that whatever caused people with Irises to be born, it started happening 27 or more years ago and the change took 5 or more years. Whatever it was, we can't say as we don't have any relevant background information, so it is best to simply assume it's the third of the possibilities you listed and leave it at that until we get more information.
Is this Iris deal a global thing or just a Japanese thing?