Ugh, I don't know what to think about this...
First of all, Ruki is suuuper cool! How come they say she's not cool?! Okay Maasa might be the coolest one (-) but ugh, what?! Ruki was the second coolest... She might not look super cool in this chapter but everyone has a delicate side, if you hurt them that much they are not going to be able to take it!
And I just want to slap Saachan! She did what she did and she doesn't even show the slightest hint of guilt! And I don't understand her, she doesn't even seem to love Ruki, you wouldn't hurt someone you love and act so calm about it, she seriously seems totally disturbed!
I don't know, is this supposed to make people upset? Is this why it's called "the feelings we must all endure"?! =.=
Yup, Ruki is cool, basically because she is a good person. She always tries to do the right thing, even if it's not always the smartest thing (e.g. she should have slapped Remi imo, and perhaps Sacchan, too).
But what they mean in the story is that cliched "cool". Aloof, tough and so on. Also assertive, which she definitely wasn't in the last few chapters.
I'm not so sure about Sacchan, though. You're right, she doesn't "show" guilt, but I don't think she feels no remorse, perhaps just in a more complex, not easily put into words kind of way. What she said holds surprisingly deep insight. She learned about herself, I'm convinced about that.
Up until the cheating, she was always a person that let herself be controlled by others. Her ex, and then Ruki. Of course we, the readers, rationally know that's stupid. But she herself didn't see that (as we probably wouldn't see it if it was about ourselves). By acting egoistically for once, she realized that this suits her more, she says so herself. However, that doesn't mean she'll treat Ruki (again) badly in the future. I read it as her realizing that she has to take responsibility for her actions and that she can only do so if she decides what she wants herself.
Look at it this way: Ultimately her just casually letting Ruki have sex with her (i.e. without "wanting it for herself") resulted in Ruki being hurt in the first place (even before the cheating the relationship was "going nowhere", Ruki felt uneasy because she had the impression Sacchan was indifferent, which she was).
Now having learned that she can also want stuff for herself, Sacchan wants to start a relationship (or "restart" in this case) out of her own intention.
Of course you could disagree with Ruki's choice to go for it, but ultimately that's her matter. I've seen this stuff in real life and actually I think this is one of the few scenario where forgiving an affair and using the opportunity to start again on better terms, better understanding of oneself and the other, can be worth it.
Even with all the drama and rage-inducing decisions characters make, I like this series. I find the characters believable (some you can realistically despise, some you just love, and sometimes one turns into the other, just like with real people) and enjoy how new facets of them unfold over time. Plus I've made some stupid and not so stupid love-decisions myself so I can relate (and yes, decisions that after the fact make you go "why the HECK would I ever have done that???")