You are unfortunately also forgetting that the scanlator had already explained in the endnote page of chapter 15 that this manga is getting an
official English translation, with no hint that volume 3 is the final volume. I don't know about you, but I don't think a manga that's about to be axed would end up getting such a privilege as being officially translated into another language, least of all English (which targets a
massive international demographic).
From the author's reply to me on the matter, it seems that this current trajectory has always been their plan from the beginning. And I should note that "we can't go back to the previous status quo, it's disrupting the world too much, so we must remerge Karin into a single individual" doesn't necessarily mean that we'll no longer see her myriad personality facets; in fact, I suspect that Karin will become a personality-swapper, switching between her various facets (with corresponding appearance changes) in a manner similar to the female protagonist of
Change 123.