Sorry for making you think about Corpse Party again, and I could be missing something because I haven't looked at Blood Drive in years, but in a scenario where Yoshiki and Ayumi cease to exist, like the true ending of BD, how and why did the rest fo the 2-9 crew go to Heavenly Host, without Ayumi to prompt the ritual? DID they go there at all? Or did Ayumi effectively undo the entire series and nothing is canon anymore?

As a note having finally pored over this for a few hours, I know it's kind of a joke that copa lore gives us all pain, but your ask helped me reach a satisfactory conclusion about some things that I haven't been able to since that very messy post about the canon, as here I could focus on these specific things, so thank you!

Before I inevitably go into a rant about the canon and start to try and make sense of everything through guesswork, the TL:DR of this is that I don't truly know, they never explained it, and they've never made any hints as to what happened according to a survivor other than Yoshiki's memory. I don't think, however, that anything was physically 'undone' that doesn't pertain to Ayumi and Yoshiki's physical presence in the universe, as there's several bits of information that at least give insight on the current universe the survivors live in.

There's really no logical reason why the Kisaragis would end up in Heavenly Host without Ayumi, because none of them knew about the charm, and I don't think Mayu's fortune-telling hobbies went so far as to find Naho's blog, especially without the influence of engaging in this hobby with Ayumi. However, everything that happened before Blood Drive still seemingly had an influence on the 'current universe' everyone is in, because those Kisaragis who died are still dead, everybody who died in Heavenly Host is still dead, and Hinoe, crucially, is still dead.

When Aiko, Magari, Satsuki, Satoshi, Naomi and Yuka leave the Nirvana, the world they return to is one in which the Nirvana and Earth beginning to merge together still happened.

[The six Pillars disappeared from the earth, leaving only giant craters in their wake.]

The actual physical existence of the Nirvana is erased once it is absorbed by Ayumi, but all the previous damage it has done remains. So unlike previously where HH survivors returned to an alternate universe where only the possessions they brought with them like their phones provided evidence that others' existences were real (not going to address Naho and others being remembered for a while after they died, this is probably to do with spiritual energy), this time they are returning to the same place. So pretty much everything in Blood Drive besides the bit where Ayumi and Sachiko go back in time a couple of hours because everyone fucking died, happened in the current universe. So yes, in this universe and stage of the canon, Ayumi initiated the charm, but since she absorbed the Nirvana, there isn't any physical evidence that she was ever there to do this, nor do the survivors remember that she did this.

There's no substantial evidence that the survivors remember Heavenly Host. Satoshi never alludes to it in Dead Patient. However, it's interesting that Naomi is training to become a teacher rather than a nurse according to Yoshiki in the drama CD, and though this could just be a change of heart through having teachers like Yui and Kuon, it could also be that she has PTSD from HH, and therefore can't follow that dream, with teaching being the closest option to what she wanted.

On the subject of Yui and Kuon, it's also clear that those who died in HH died... when they died, rather than their bodies suddenly being found newly dead in the real world, because Kuon still seems to have been the assistant homeroom teacher for 2-9 for a little while.

[Next to the photo of her classmates, another photo stood on her desk, taken at Mochida's house.]

(Note for the above: there's some ambiguity with perspective in this screencap but I don't think it's relevant).

Obviously this scene still happened in the canon for this world, and Naomi's eye remains injured. This raises a lot of questions, because her original injury was due to her looking at a picture of Seiko from their old universe, and so whether everybody else in this universe remembers 'forgetting' about those who died in HH and the issues Naomi and her mother had regarding Seiko's existence still applies is kind of crucial to my personal theory about the survivors' memories regarding Heavenly Host.

In the instance that everybody is aware they forgot the existences of those in the Nirvana, which, realistically, has to be the case if the survivors of Blood Drive returned to the same world in which Naomi received her eye injury, and Yui was inexplicably replaced by Kuon, it is a given that Satoshi, Naomi and Yuka must remember the most recent loop in which they lost their four friends in Heavenly Host but nobody else remembered, as well as the point in which people began remembering. I imagine the most practical explanation for this for the rest of the population is that due to the apocalypse™, a bunch of weird paranormal shit happened, the pillars disappearing may as well be like how their memories disappeared for a short time, and they just have to accept that they didn't previously remember a series of things that actually happened, and that was a practical lifestyle for them. With this kind of public mindset, it seems more plausible that Yoshiki was able to seek help from Hinoe's colleagues, give an explanation as to what happened and have them believe him, because whether they choose to believe how it happened is up to them, but they cannot deny that something did indeed happen that led to all these instances in the real world, with the disappearance of the Nirvana from the sky unexplained to everyone else.

For Satoshi, Naomi, and Yuka, it is plausible that huge chunks of their memory are missing in terms of who introduced the ritual to the group, how they managed to find a way out, and why exactly they returned to the Nirvana in Blood Drive, but they still remember these events, and are aware in the same way when they compare themselves to the rest of the population, that they too are unable to recall some really important details about what they went through. And there's nothing they can really do about it, knowing how others were prior to the reinstatement of the memories of HH's victims.

It is additionally possible that they remember that there were other people there with them, but they remember no details about them whatsoever.

[...and somewhere in this world, I'm certain she must still bear it, protecting us all from deep under cover of night.]

[She's been erased from our memories, however, so... wherever she is, there can never truly be any communication between us again.]

It's a bit iffy using Aiko's monologue at the end as evidence, but these lines in particular seem to indicate she specifically has memories of a girl sacrificing herself to stop the world from ending, rather than just narrating for the players' context. She might remember more than the other survivors due to her spiritual abilities, but I feel like this still indicates that the other survivors at least are aware their memories have been severely tampered with as a result of someone else's existence being erased, and they simply have to live with unanswered questions about their experiences.

As a sidenote, I do wonder how it was determined that the Nirvana's victims were dead, rather than missing people. Perhaps their bodies were returned to the place where they performed the charm or were taken to HH before it broke down. I find it hard to believe Satoshi and co. would give a thorough explanation about who died, people like Morishige's father believed it, yet they never had contact with one another (especially considering Seishirou is a neuropsychologist. I'm certain if he knew how his son died and who the survivors were he'd be doing research on them for sure).

[I would have been consumed along with (the) Nirvana into Shinozaki, and disappeared (there's a 10-image limit on this, so I couldn't include the screencap containing those last two words - sorry!).]

However, it's also suggested that everything within the Nirvana does not return to the real world, since Yoshiki believes he wouldn't have been able to leave without the Everafter Stones, so maybe their bodies were never found, but it was just assumed during the apocalypse™ that everyone missing was dead. But this is also Yoshiki's assumption of what would have happened rather than fact, so maybe this isn't the case after all.

I also wonder why, if Magari had contact with one of Hinoe's colleagues, Yamamura, she didn't ever think to revisit her. She may not know of Ayumi's existence and therefore not know why she contacted her, but the not-knowing should be a good reason why she should investigate. But perhaps there's no physical evidence of her contacting Yamamura, and she just jumped her in the street or something and forced her to deliver Ayumi's next course of action.

And to be honest, the actual status of Ayumi and Yoshiki in this world is still a bit difficult to parse, as someone relying on unofficial translations.

[But thanks to the existence of the us that we don't know, there aren't any bothers (problems) in daily living.]

Parts of the translation on the drama CD imply that versions of Ayumi and Yoshiki did exist in the current world, but that they were rewritten so they didn't cross paths with anybody they previously knew... which contradicts the idea that everybody returned to the same previous world.

[Instead of just coming back every once in a while, come back properly.]

There's also implications that Ayumi is slightly more conscious than she appears at times, but this is inconsistent. And I also hate this for the canon just because she shouldn't really be able to recover from a vegetative state for the sake of her actions having meaning and serious consequences. The other drama CDs aren't necessarily canon though, so I'm choosing to ignore it for the most part lmao.

I don't know if I can be bothered to write a conclusion to this, other than that most likely everything from Blood Drive* happened in the current world Corpse Party (aka Dead Patient) is set in, and that any inconsistencies have just resulted in survivors conveniently forgetting the details, but knowing they are missing memories, and they have to deal with that for the rest of their lives, so the Kisaragis' way of going to Heavenly Host and the results of that are presumably the same. But this is all guesswork from the limited information we have.

*As I tried to conclude in this post I don't believe the specific turn of events in any content before Blood Drive can be considered canon in this timeline, especially due to BD's dates that it occurs during, and that all we know for sure is that the Kisaragis went to HH, and four people from their group died (we don't know how). Though it should be said that I don't agree with absolutely everything I said in that post anymore, in light of the DP drama CD and revisiting the last part of BD (even I still have age-old copa content I haven't yet gone through haha).

DP drama CD translation (English captions)