One important thing is that, judging by those two volumes that we've got, and by the latest tweet from the author where they say "which heroine will appear next time", the story is being told as self-contained substories narrated from an uninterrupted POV of a heroine, and switching between Yami and Hikari would break that. The big reason for the impact that was chapter 21 was that Hikari's POV did not let us see all the clues for what they were, so POV is an important narrative device here.
It is not unlikely that Hikari might chose to not confront them at all and quietly back off, and so we wouldn't get any of these flashbacks. But even if the events will happen as you describe or close enough, having Yami's story told in form of chopped up flashbacks as opposed to a self-contained volume would shift the narrative weight from "Yami and Hikari are equal heroines" to "Yami is a side character", which does not seem to align with author's intentions. As for the possible repetition, now that we know the context, the author has all the tools to condense it in like one page of "they dumped their backstory onto me", and we will be able to see Hikari grimacing in one chapter as it happens in real time instead of multiple chapters interlaced with flashbacks. We were shown the context, there's no need to tell it to us.
Way to dismiss all history books. Sometimes context IS the story, which I believe to be the case here.
Yuu knows, he was a direct participant. In fact, he knows more about the events of these nine chapters than he and everyone else combined excep knew about the events of the first arc. I say it again, I don't see how that is any different from the first 20 chapters.
Are events of this arc hard to digest? Taking spoonfuls of watermelon while eating rice sounds even less digestable to me. Like, seriously, jumping back and forth between POVs - and as we've established, POVs are important here - seems conceptually more confusing to me, for one, the author would constantly need to reestablish whose POV it is.
Kills how? Did this arc make you forget about the events of chapter 21 already?
What exactly are "momentum" and "pacing"? If it is a measurement of significance of the events being told, the manga was a slowburn during the first 20 chapters, and then it was imparted the momentum of drama in chapter 21, and the dramaticism was retained throughout this arc to the point of being melodramatic. If it is a measurement of how linearly the story is being told, well then it's a stupid measure, because a story does not need to be something that is a sequence of events progressing towards the future. Non-linear stories are usually introspective, where events as they occur are mere devices existing for the sake of giving context to personalities. In introspective non-linear stories, the questions and the intrigue are usually in the vein of, "how do events in the story affect them personally" as opposed to "given their personalities, which events will occur next" is a typical linear story. You're fixated on "what happens next", but that's not what the story is trying to be.
