Speaking as someone with OSDD (like Dissociative Identity Disorder but with no memory loss) this is actually a fairly interesting and comprehendable exploration of someone who has a somewhat fragmented mind (but not quite all the way). It honestly sounds like OG Karin had something akin to three stooges doorway syndrome, there were so many nuanced aspects to her mental space that they all got stuck lodged all trying to work at once.
We joked about this before but honestly, this along with the "there was no singular, tragic event" line honestly DOES make us lean a little towards Karin having undiagnosed OSDD 1B. Most people don't realize that C-PTSD, or just repeated mild trauma, can cause OSDD. We help a lot of people with the condition and they often assume they need a bit tragic event to be "plural", when it's often the opposite.
At the very least Karin feels somewhere on the spectrum. And, being a teenager still struggling with her identity, found a way to quite literally personify each of those aspects into their own beings. It's honestly a lot easier for us to comprehend this manga if we interpret each Karin as a previously unidentified alter being made manifest in the flesh. Honestly, Yuu giving them each nicknames is also what we would argue as evidence to this idea.
So. Where to go from there, then? How should Karin grow from this conundrum, of her brain literally wanting to be many different people at once? Perhaps even being multiple people at this point? Honestly... and again, this is speaking as someone deeply experienced with people who are literally that irl just without the body splitting:
We think they should be in one body and take turns.
Alters are like people. They grow, change, and adjust like anyone else. The reason Karin was struggling so hard was her different aspects were all fighting to come out at once. And simply fusing them into one singular being has clearly been portrayed as deeply harmful to Karin's mental state. What needs to happen for anything to happen is to treat "Karin" like an overall shell, a car to drive for each of them.
And again, if you doubt us, we're speaking heavily from experience. The most well adjusted systems we've seen have let their alters grow and change by giving them time to see who they are. And (outside of an expensive fashion bill), it's kind of the only way we see Karin managing to come together healthily while still keeping all the offshoot Karin characters as individuals still. If it's really still gonna go, maybe the charm will keep spawning different aspects of Karin that need to get shlormped back into the "inner apartment" of Karin's head, to keep the fantasy aspect, haha.