I found it funny how Ririko's ending was kind of like the harem ending as well. Mamoru, with the permission of Ririko, is pretty much getting everyone pregnant except for Kotone who feels left out. But of course, Ririko changes that too haha.
Although, I do think Yabui stays in this ending... I mostly skimmed through the ending and forgot most of it haha. But I think he just ended up helping Ririko's agenda to get everyone pregnant (with mamoru of course). I also remember something about them using something on Mamoru (probably to keep his sex drive up).
The harem part (with non-stop babymaking, Yabui's fertility drugs,etc.) in Ririko's ending was a dream Mamoru (after taking down Shinohara and passing out) has showing what would happen in the future if Ririko is left as she is. Ririko has an uncontrollable sex drive (which also awakens her yandere side) due to being jacked up on aphrodisiacs 24/7 because of Yabui's medication. After Mamoru wakes up from the dream in Itsuki's room, he tells Mamoru he's going to leave on business trips indefinitely but offers him a special medicine. The medicine would make Mamoru's semen a super aphrodisiac for women, thus countering Ririko's sex drive, but it would also shorten his lifespan. Itsuki has already taken this medicine as shown in Kotone's Itsuki NTR end. Mamoru chooses to take the medicine in order to support Ririko. Right before Itsuki leaves, he also orders Yabui to leave and pays him out.
In Mamoru/Ririko's final sex scene we can assume Ririko will be less yandere and more deredere since Mamoru can easily satisfy her. Of course, Saya and Ririko are still pregnant and it's reasonable to assume Kotone will eventually be given a chance, only that it won't get out of control like in the dream.