Chizuru's grand three moments where she acted outside of her own interests are as follows
1. Coming over to Kazuya's rescue when his grandmother came over and wanted to see Chizuru.
2. After Kazuya's grandmother left, she agreed to let Kazuya rent her at specific times for the sake of his grandmother
3. Pleading with Mami to reconsider her stance on leaving Kazuya
Why are these the only three times (to my recollection) that Chizuru has acted selflessly, and in opposition to her own interests? Because after Kazuya rescued her from drowning, she fell in love with him, and every decision since then has either involved in her acting in the interest of getting keeping her job a secret (see Chizuru forcing Kazuya into a relationship with Ruka, despite Kazuya giving a firm no), or Chizuru wanting to get closer to Kazuya (see the dream date or birthday arc). So much for Chizuru's "selflessness".
On Kazuya and Chizuru having never gaslit Ruka:
> "first of all nothing actually did happen between kazuya and chizuru those times"
First example, Ruka catches Kazuya cheating, Kazuya had previously told Ruka that he was "busy". Is caught on a date with Chizuru, confronts him about cheating, denies it and tells her that he is simply getting life advice from her.
Second example, Ruka catches Chizuru leaving Kazuya's apartment after they had a meeting discussing movie, and had dinner together. Your argument that "nothing actually did happen" almost holds up... but we get to see Kazuya's thoughts where he says that their relationship is changing, and wishing the moment could last. We can infer from Chizuru's "Fuuuuuu" after leaving, that she thinks the same. Yet when Ruka confronts them about their relationship changing, Chizuru tells Ruka that all is fine, and they are no closer than six months ago, and use the movie as an alibi.
Yes it is true Ruka is guilty of gaslighting Kazuya in Ch 157, and I find that detestable. However, I choose not to turn a blind eye to Chizuru and Kazuya's misdeeds, nor will I try and excuse any of their deeds. While we're at it, you brought up Ruka drugging Kazuya in a attempt to have sex with him. I'll give you the benefit of the doubt and assume that you don't know that alcohol cooks out the food during the cooking process, in fact it takes a very low temperature to start cooking out alcohol, making it practically impossible for Ruka to have drugged Kazuya unless sufficient alcohol and concentration of alcohol was used.
As for the sex part, I believe that sex was a tertiary objective. Why? Because Ruka never goes about the same way twice, more on that later. Ruka already offered to have sex with Kazuya, and was refused. Now she had a new method, the ultimate girlfriend/housewife method personally tailored for Kazuya, come home from work, cook dinner for him, relax and watch a movie, and possibly have sex. Why else would she have forgotten about the condom if that was her primary objective? To add to that, why would she walk away once Kazuya told her that he was moments away from giving in to his desires and sleeping with her?
Ruka does greatly consider Kazuya and his feelings, and wants him to be happy, however we also know she has an impulse problem that, like her toxicity, she has been making great strides to address and correct. Just look at the hot springs arc. Ruka says she wants to go to a hot springs, Kazuya doesn't say no, but recommends a place nearby. Ruka considers Kazuya first, and agrees to his request, despite it only, once again, acting to her own detriment.
> "[If you love somebody] you want the best for them and you want them to be happy."
Yes, but it's not easy. I dare say I bet you've hurt someone you love, does that mean you don't love them? No, of course not. Be it negligence, or selfishness, you didn't intend to hurt that person, and you still very much love them.
Now let's turn to Chizuru. does she love Kazuya? Yes. Does she want the best for him? Yes. Does she want him to be happy? Yes. Wants to help Kazuya in his screwed up circumstance? Yes! Does she act? No. How is it okay for Chizuru to know that she could single handedly solve the Ruka problem, make Kazuya and the Kinoshita family happy, all by accepting her feelings for Kazuya and becoming his girlfriend, yet she refuses to act? She is leading Kazuya on, and has been for awhile, she knows Kazuya likes her, but for no reason what-so-ever, she chooses not to reciprocate. She even goes out of her way to lower Kazuya's self esteem, like when she ignores him in the hallway just a few chapters prior.
Going even further, Chizuru actively sabotages Kazuya's growth, and cripples his mental health and emotions. Let's take a look at a few examples of how she does this, "Tell your grandma we broke-up; Don't tell you grandma we broke-up / Don't go out with Ruka; Go out with Ruka / Do you love me?; Of course you don't love me, your only a client. Definitely a perfectly healthy line of communication and the embodiment of a non-toxic relationship. 174 chapters in and she answers for him, and later ducks his confession, talk about someone hurting his self-esteem. Let's not forget that it was Kazuya who manipulated Chizuru into being his "girlfriend" in the first place.
This brings be back to Ruka. For all of Ruka's toxic actions and behaviors, at least we see that Kazuya and her communicate on a level that each other understands. Ruka directly asks for sex, Kazuya says no, Ruka understands, doesn't go that route again. Ruka wants to stay the night and is open to anything that might happen, Kazuya communicates that he is uncomfortable with this, Ruka understands, does not push sex or a sleepover in any regard again. This is not to excuse Ruka's actions, but to point out that she forces Kazuya to grow by making him confront parts of himself that are needing attention. Kazuya grows as a character from these interactions, and sets boundaries, Ruka recognizes and respects these boundaries for the most part, with the exception being the kiss (sexual assault), and from this scenario, Ruka learns that she must do even better to avoid hurting Kazuya in the future. Chizuru can't even communicate to Kazuya a simple question without either indirectly, or directly answering it for him. Even beyond this, Kazuya and Chizuru just communicate in different ways. For Kazuya, having a bouquet of flowers and chocolates is his way of saying "I love you." For Chizuru, her taking you to batting practice is her way of saying "I love you." Neither of them understand each other, and if you have a hard time just communicating to each other that you love each other, imagine how hard it will be to communicate that something is wrong in a relationship. A long term relationship between the two of them wouldn't last.