I really like how this work distorts the panels, adding in frame-breaking images or little background designs that accentuate the atmosphere.
Okay some general "facts" can be made about what's going on. Why or how those things happen are ridiculously unclear though.
Facts: Seki masturbates here (pointing this out because I completely missed it at first). She also sleeps with Kagerou on page 13. If there is any point at which Sekibanki's narrative becomes unreliable, it is after page 15. Wakasagihime somehow ends up in a situation where a group of humans are attacking her with very sharp weapons. Sekibanki thinks she killed Waka, when Waka is still alive (?) but heavily injured.
Other random observations that are interesting but don't know how to make sense of:
i. Hand reaching for the book on the title page could be anybody given the author draws all the characters with sharpened nails. Otherwise I would have said Kagerou's most likely. It could be Waka, because Seki's sleeves are always darker/black for the most part.
ii. A specific (white?) flower in page 9's background drops its petal. Probably cherry blossoms.
iii. No deeper meaning, I just went wtf when I realized there's cum on Seki's inner thighs here. Really showing her as depraved/tainted.
iv. Page 15's vines were strange, because the "thorns" are rounded. So not the typical rose vine. Might be budding cherry blossom branches? shrug
v. Bare branches on page 16 and leaves falling off. Could be to depict the season. Bell crickets are mentioned there too. Animal Crossing wiki says they come out during autumn lmao, but couldn't find any other sources.
vi. Eirin wrote nothing down? Or are those blank spaces for another session.
vii. Impossible to tell if Seki's thought at the bottom of page 18 is only to herself, said out loud, or trustworthy. It is jarring though, because I thought her story recounting was over.
Most interesting possibility to consider for me: The medicine Seki says she got from Eirin are dream pills. That's the medicine Eirin is canonically known to give after all.
a. They are butterfly dream pills (good dreams).
b. They are nightmare dream pills.
A could be true if Eirin is genuinely trying to help Seki. Doesn't seem to be working though if that's the case.
If B is true, then Seki's nightmares begin on page 14 in line with the abundance of pills appearing in the background. She thinks the resulting events happened because of her when they in fact weren't.
There's lots of ways to twist the narrative, since it's intentionally confusing. I'll go with the one I believe in the most cause describing all of them would take too long.
Seki is not at fault. Her guilt for not being able to protect Waka from the outside world, the goal she'd attempted for so long plus her prior self-hatred, leads her to break and take all responsibility ("your mind is just trying to deal with the exhaustion"). Eirin wonders when "[Seki] will open her eyes" ie. realize the truth/wake up from her nightmares. Even though she says there's no forgiveness, Eirin explicitly suggests otherwise, even appealing to the wounded Wakasagihime.
Now that I've written all of that out, you know what, that makes for a surprisingly simple explanation lol.
Food for thought addendum: Eirin calls Waka sleeping beauty. The nice implication is that Seki is still her knight. The not great implication is that there is an "evil" out there. Even though there's no given motive, it's possible Kagerou actually was "evil" and carried out what Seki dreams of/remembers. Or it could be Eirin. This would support the medicine being nightmare pills theory.