Ok, rereading more carefully, I think I know what is actually going on. The entire chapter is from Airi's point of view and starts with "I don't really remember what happened that day". This means this chapter is only what she remembers. The weirdly cut panel on page 2 and all the censorship are her memories being messed with. The cause is the chocolates which, from context clues, we can deduce are supposed to produce some sort of hypnotic effect, though it seems to only affect Airi.
With a bit of educated guesswork, we can figure out what's been censored. Page 3 is how the reader is intended to find out that they're talking about hypnosis. Page 4, Nagi looks for instructions on how to give suggestions to a hypnotized person. Fumi directs her to a site (QR code?) on the back of the chocolate box, then warns her that something bad could happen if she doesn't undo the hypnosis correctly once she's done. The weird text outside the panels are meditation instructions to help the hypnosis work. Page 5 is when Nagi notices that the chocolates actually worked on Airi and when all the hypnosis actually took place.
We can assume that Nagi's hypnosis instructions were to forget that they talked about hypnosis that day (thus the censorship), and to fall in love with Nagi. However, she appears to have not undone the hypnosis afterwards correctly, thus the turn to horror at the end. The horror likely leaked in because they had just watched a horror movie before.
