Hmm didn’t put much thoughts into how or what exactly others are doing during resets, I always assume they are having their customized nightmares, something similar to what Maya had during 4th reset (forgot if Ayane ever described how it felt before waking up). The “inflict extreme harm” part is something I simply perceive as the hard reset if he keeps fucking up the trivia, as Sensei is the anchor to everyone; but what you suggested is possible.
Yeah, I could see them having their own customized nightmares. Maya seemed stuck in her nightmare without Sensei's help though, so I'm thinking she went through something different this time. More of a "personal trial" like Long Maya said instead of an old memory. Otherwise I don't see how they'd get to the roof days before him. It is possible no one can remember the trials though, so it's unlikely we'll get much confirmation about it.
As for Tsuneyo, since Sana also remembered having a Ayane-less sleepover in
Sweet Vermouth, I assume the whole thing worked as that sleepover still existed but false memory has been inserted into Ayane (and perhaps only her since she was the only girl at the time that was somehow "involved enough" to deserve a memory alteration).
Same logic applies to Maki being the only person remembering Makoto's first breakdown after
Bluejay; the world mechanism rewrites only the key person's memory, but seemingly cannot undo everything and/or is not flawless enough to also do something to others' memories to make everything seamless (i.e., Tsuneyo or Sana or Maki aren't being special or immune during moments like these; they were just in a way close enough to a key event, but not involved enough to get mingled by the mechanism)
With all that being said though, I still have my doubts if that sleepover still existed or not, considering Sana was also in her dorm room and not at Sensei's house when Sensei went and looked for Ayane in
Before the Sun Comes Up.
Granted, judging from the conversation the event is between Sunday night to Monday early morning, while the sleepover is on Saturday night, so it might just because it's basically a different day now after a reset, but the idea of time passing during a reset isn't all that certified (while Maya did mention how long sometimes she had to wait on the roof but I am not too certain if those passed time carried over when the reset is done).
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.
However, notice that Tsuneyo says things "ended rather strangely":
Meanwhile Sana mentions nothing weird about the slumber party itself, but she seems unsure of when it actually happened:
Her being in the dorms, and Sensei coming back to a house with none of the girls (besides Ami) there, kind of implies it didn't happen in this timeline. Much like how Makoto jumped off the roof, but then "wakes up" back on it like nothing happened, albeit obviously aware something did.
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.
I think Maki's situation is more of an exception here, as I don't think time was actually rewritten there. Makoto's memories were presumably the only thing actually affected, and just no one besides Maki really cared to talk about it. I think Makoto herself just didn't remember it. Unless someone like Ayane seemed to forget it, which I may have missed.
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)