I'll write up what I think is happening in chapter 16, since it's fairly confusing. The manga doesn't expect you to know all this yet, so if you're confused that's perfectly fine. But there's been enough hints that some very reasonable guesses can be made. This is a dense manga and I think on a re-read you'll notice many details that bring this all together.
An abridged(lol) speculative summary of the story so far:
Aizawa and Michi met and became best friends during her brief time at school. Aizawa was busy as an idol but also sick with what might have been a curse. She likely met Michi through her investigating this curse, which would allow most of their meetings to happen outside school. This is important because practically no one has seen them together.
Michi was a fledgling exorcist and connected with Karasuma. Something happened at the Obon festival 2~3 months ago, where she took Aizawa, and Aizawa died a few days after. This is one of the hardest periods to guess now, but Michi seems to have drawn up the marks around school to bind Aizawa's spirit, establish a barrier that kept powerful ghosts out, and then lost her memories. It's important to note she was trying to protect Aizawa from something, and it's debatable whether losing her memories was a result of the binding or part of the same curse that killed Aizawa.
There is also something preventing Michi from acknowledging Aizawa. If she tries to speak to her, reality breaks down, Michi suffers physically, and her memories are erased again. (However this could be a safeguard of Michi's creation, which I'll get to later.) There is also a younger version of Michi who appears when she's unconscious and protects her/Aizawa. Young Michi seems to be some kind of independent guard system possibly embodying Michi's unused/sealed exorcist powers. But whatever power she was given is finite and running out.
Now that her powers are fading, the marks around school are also disappearing, including the one binding Aizawa to Michi. This could potentially limit her to the classroom again, where the original mark was. But the barrier is also broken, allowing stronger and more dangerous ghosts through. The ghost haunting De:Lphinus seems connected to Aizawa, since it reacts to her being forgotten and Non thought its voice sounded like Aizawa. It's currently possessing Ange to reach Aizawa.
As a lingering spirit, Aizawa has an unfulfilled desire trapping her in the world of the living. Hers is to be an immortal idol, never forgotten. Being forgotten triggers an extreme response, and she struggles to control her emotions. In chapter 15 she unconsciously possessed Kasumi after Michi called her her best friend, likely because Aizawa was Michi's best friend before. In chapter 16 Michi denies ever knowing Aizawa, causing her to become grotesquely misshapen, her features dissolving away.
While this frightens Michi out of her mind, I don't think she was in any danger. Rather, Aizawa was truly dying here. Being forgotten by the person she cares about most was too much for her spirit to survive. Different to what happened in chapter 12, this started with Michi denying knowing her, not when she acknowledged her. If it weren't for Young Michi Aizawa might have slipped away into nothingness.
Which brings me back to Michi being unable to speak to Aizawa: if Michi knew this could happen, her being unable to acknowledge Aizawa might be a rough safeguard to prevent Aizawa from finding out she'd been forgotten. Michi didn't seem to have a plan beyond keeping the status quo as long as possible.
Sayaka is another spiritualist at school with no connection to Michi. She has no idea what Michi and Aizawa have been through, but recognizes Aizawa is a ghost and that Michi made the marks binding her. It's purely speculation, but my guess is she plays dumb when it comes to ghosts to treat them like normal living people. This would prevent cases like Aizawa from being shocked into formlessness or worse. I think she's still not the brightest though.
It's also worth mentioning Aizawa seems to remember everything from the start, unlike Michi. She also likely knows Michi can see her, or at least does after this chapter. Though I'm not sure if Sayaka is aware Michi can see ghosts.
The play at the start of the chapter is a blatant parallel to Michi and Aizawa's situation. Aizawa came back out of a desire to be remembered. If Michi fails to remember her, terrible things happen.
Karasuma is a professional exorcist who perhaps took Michi under her wing. She knows the whole situation with Aizawa and recognizes her as a unique and potentially very dangerous ghost. Her visit in chapter 13 likely came after learning that Aizawa was causing harm and on her way to turning wrong. But she admits there's still hope for something better with Aizawa, which all depends on Michi's influence and protection. The end of the KogaFes arc will be Michi's big test on whether she can protect Aizawa, both from herself and predatory ghosts.
But all this pales in comparison to the revelation Seta indeed has 2 moms.
thanks for the summary id kind of forgotten all of this
