Seems to me like Sakiko is just toying with Chika, forcing her into a (physical) relationship and using her to get away from her supposedly boring everyday life.
It looks like there will be one more Sakiko & Chika bonus chapter later on.
And it will be 15 chapters in total.
Ah Erika
btw they are probably at Todoroki Valley, which is in the middle of Tokyo despite looking so remote.
https://tokyocheapo.com/entertainment/todoroki-valley-tokyo-secret-escape/