I actually think it might be the other way around. Sana remembering the slumber party is the false memory, just like how Makoto remembers killing herself but it's treated as hallucinations in her mind. Although I suppose it's more like retconned memories than false ones. Time gets rewritten, but memories aren't rewritten competently, leaving behind memories of things that couldn't have happened. Similar to how people tend to overlook how long it's been, even when noting it's been years or more than one of the same Holiday within one year.
For the record the false memory I referred to is any memory that conflicts with what Sensei remembers (or at least what players saw), regardless if it later on got corrected or rewritten or not. So Makoto's no longer existing suicide attempt is real, the may-or-may-not-existing 3rd reset sleepover is real, years of memories compressed into one single semester are all real; they just don't make sense, but they're real.
(but by my own logic this means Tsuneyo knowing Sensei's first day speech would be a false memory, something I am not too sure how to think of haha)
Meanwhile Sana mentions nothing weird about the slumber party itself, but she seems unsure of when it actually happened.
I read this in a much simpler way, that Sensei starts being weird about something happened so recently, and Sana merely confirmed that what Sensei thought to be a no longer existing sleepover still happened, with a tinge of "are you okay?" tone rather an uncertainty. I'd probably respond the same way if someone suddenly stares at me and has an out of nowhere existential crisis over a sleepover.
Tsuneyo on the other hand mentions it ended strangely, implying it's not a false memory, but an actual one. She didn't seem to have trouble placing the memory or what happened back then. Albeit, Tsuneyo can apparently remember things she's not even around for. So, who knows what's going on with her memory.
"Ended strangely" is indeed intriguing; unless she was just referring to Sensei leaving the sleepover in the middle of the night after getting a call, but I am open to the idea of her witnessing something when a reset takes place. As for her "remembering" something Sensei said only to the first 10 girls (well, 9 because Yumi wasn't there lol), I think this might be less about Tsuneyo but more about this new game+ mechanics that Kumon-mi uses that has an effect on her; if only we can ask Molly the same thing and see if she also remembers a speech that she wasn't there to attend.
Btw, from what I understand, the Slumber party was on Saturday night, but by the time Sensei got done talking to Ayane and made it back home, when the reset starts, it was Sunday (after midnight). By the time the reset was over, and Sensei checked up on Ayane it was around 3 am:
Sunday:
In other words, Sana, Ami, Molly, and Tsuneyo should have been at his house while he was talking with Ayane here, but Sana's definitely not. Monday wasn't really involved in the third reset, besides Sensei mentioning he'll be a better teacher tomorrow, then rephrasing it as him being a better teacher today technically, since he's deciding to be a better one now. (Which likely lead to the confusion)
I thought Sensei questioning Ayane was Sunday-Monday (aka about a day after the sleepover) and not Saturday-Sunday is solely based on her saying "...even if it's almost time for school" during a 3am exchange, so I quickly went back to a 3rd reset save to check on exactly what day it is.
Sensei went back home to sleep at the start of Word of the Day, which takes place after meeting with Ayane the first time on the roof on Saturday night, then it transitions to 3rd reset BS puzzle part of the event that takes place on Sunday. Then, what followed immediately after 3am exchange is (bad uncle route)
As Light As Air on Monday.
Though what exact day it is probably bares little relevance, this is a very roundabout way to say that Sana
can be at her dorm when Sensei checked on Ayane because the timepoint of that may-or-may-not-exist sleepover has already been in the past; Sana's presence there isn't a valid evidence to prove that the sleepover got rewritten into never happened (i.e., not a "Weren't you supposed to be in my house?" kind of encounter). The
technically today to me is as literal as Sensei revising himself since it is not Sunday midnight but Monday early morning. Once again if only we can ask Molly again to see if she remembered anything about a sleepover lol.
But I do notice that Ayane said "And I can't really figure out why Sana would be here if there actually
was one [sleepover]" a bit confusing though, a piece that doesn't fit well into what I proposed (so here you go my weak spot!). And I think there is still a possibility that the sleepover became "unhappened", but current findings aren't concrete enough to support it.
