Karuta stops Kagerou from going back to the past saying that she doesn't want him to be alone, that together they will a find to stop Inukami and they'll all age together (god I nearly shipped Karuta x Kagerou)
Watanuki suggests to send a letter to the past to warn their past selves. Everyone is surprised but they agree.
Renshou starts getting nostalgic and says that when he opened the letter he wrote back then (the one in anime episode 12, manga chapter 9) he was surprised about the random contents it had. Nobara asks him if he was the one who most suffered and he replies her that he was able to move on during these 23 years since he had other friends and even got married and had children! But even so, he could not forget them.
Karuta tells Watanuki about their past lives (he still doesn't remember it). She isn't scared of reincarnating again because she read the letter that past-Watanuki wrote and it said that he loves her and no matter how many times they will be reborn, he will always be there to protect her
An embarrassed Watanuki denies all that
Ririchiyo is in the hospital recuperating from Kuroe's fight and Soushi is crying besides her, asking her why he protected him; if he did it because he looks like the previous Soushi. Ririchiyo confesses that indeed she doesn't love him, she stills loves the previous Soushi but she treasures him and that maybe if they had more time she would ended up loving him to.
The scenes changes to the past (anime episode 12, manga chapter 9) where Renshou is digging a hole. The chapter ends when the cook's son finds a time capsule (the one sent by their reincarnations from the future).
Inu x Boku SS, end of the Second Chapter "if"
So aaaall these past chapters are like some sort of AU where they fail to stop Inukami. It also means that we're back with the old characters (I can finally ship again Karuta x Watanuki and Nobara x Renshou because in the "future" their age gap was too wide)