The player only controls Ian and Lena's actions, you can't control their feelings. Depending on your choices, Lena suffers from inner conflict and feelings of guilt. You can see it multiple times throughout the story:
That is true, however those feelings don't come across as genuine feelings of Lena, but rather a desperate attempt to convince herself that she's not a bad person so she must feel guilty (when she totally didn't when she was making a decision). But if she's guilty, why does she keep repeatedly doing the same thing over and over, thus feeling more guilty? If a person consciously keeps doing a bad thing (if she thinks about them, she's conscious of them and has agency to stop herself), even if they feel guilty, they kinda become a bad person because they change their actions shape their personality. Lena becomes a bad girlfriend to Ian, and a bad friend to Louise. I guess my question is, if Lena understands that she's doing those bad things behind Louise and Ian, can she really claim that she cares about them at this point? It's just in my experience, people who are not diagnosed with some kind of disorder or mental handicap, don't keep doing a really shitty thing if they really don't have a good excuse or are motivated by a strong emotion (lust is not that). You don't keep kicking your friend in the back, and then ask how they're doing if you intend to keep kicking them. That's behavior of a psychopath. Usually when you treat your loved ones badly, it's because something irritates you about them, and you want to act out, which means your relationship becomes worse with them. It made sense for Ashley to want to bully Eva if they weren't friends anymore. And at this point it would make sense for Lena to bully Louise or treat her like shit out of her resentment caused by her lust for Jeremy (she constantly refers that she was envious that Louise got to enjoy his dick all the time). But she does the opposite which makes Lena's actions seem like they happen in a bubble and have no consequence for her personal moral corruption. She's not only hypocritical in this sense, she's almost bipolar or borderline, because her actions and thoughts don't align. It's like Lena the "Louise's Friend" and Lena the "Jeremy's Slut" are completely different people that take control based on whether Lena feels horny at the moment or not.
It's true that Lena's attraction to Jeremy is purely sexual, but we can't ignore the underlying context. Jeremy can't be replaced by a random character. For those who like cheating and corruption, cheating on Ian with a random guy is pretty boring/entry-level when compared to cheating on him with one of his best friends (who happens to be in a relationship with one of Lena's friends and roommates), which is far more kinky.
You summarized it perfectly, but let's look at what you said. The context is that the players want to be naughty. "Those who like cheating and corruption" which Lena is none of those things. Nothing in Lena's personality would make her so weak to fall for some convenient black dick and betray her fairly close friend while somewhat remembering that she's actually supposed to be in love with Ian. There's a direct conflict between what the player wants Lena to do and who Lena is as a character (not a blank slate with a baggage). Players corrupting Lena results in such a mess where Lena can easily participate in all debauchery, then have obligatory guilty feelings that feel fake given what she's done, and then suddenly act surprised and outraged whenever someone does remotely similar but she ends up being on the receiving end of those actions. Blaming Ian for dishonesty but easily keeping secrets from him, blaming Ivy for doing business with Axel behind her back but then doing something way worse with Louise's boyfriend.
One path however avoids these problems. Let's look at Mike for example. It is a very controversial path due to Lena's background trauma from cheating, but one can sort of understand how not knowing Mike's girlfriend personally somehow makes it okay in Lena's mind to be the other girl, because she's not the one being hurt in this scenario and it's thrilling for her to be the seductress and be the one with power in a relationship for a change. The difference however, is that Lena in this path
enjoys doing a bad thing, she barely even thinks about Mike's girlfriend, disrespects her during sex with him, so not having guilty thoughts about being naughty helps sell this path as believable. Lena is yet to face reality of her actions. While she might act like a thirsty bitch, in Lena's mind she's not responsible for Mike playing along.
Being involved with Jeremy on the other hand is a separate can of worms that the quotes you mentioned demonstrate very well. Most of them are spoken by Lena in conditions where she catches herself doing a bad thing, acknowledges that it's bad, and ends up doing even a worse thing just because... she's horny? Why does Jeremy in particular make her this stupid horny, and why is it so hard for her to fight that lust or deal with that in healthier ways? Such behavior makes Lena feel very shallow, except she's anything but shallow in the rest of the game, so, what person Lena really is? The traitorous cock-hungry bimbo or the intelligent and empathetic person who hates injustice? Perhaps the problem is that the game wants to sell the fantasy of Lena seducing Jeremy without actually considering that it goes against Lena's principles? In my opinion, it would make more sense if Jeremy was the one who initiated flirting with Lena, and Lena would feel rush of flirting back, acting on her desire even when she originally didn't plan for it. That would make it easier to swallow that she can both want to fuck Jeremy and feel sorry for Louise. But because she hunts for Jeremy, seduces him with clear intent, then it's really hard to swallow that Lena is really such a disgusting individual with absolutely zero self-awareness or defined moral standing.
In GGGB, player's influence on Ashley was tracked via Bad points. She could become a murderous psychopath, and somehow it would still be "logical", because a murderous psychopath wouldn't feel bad about cheating on her dear boyfriend or care that her ex-BFF committed suicide (Oh no! Anyway...). Tracking Ashley's morality helped justify Ashley's crazy actions and prevent her from contradicting herself with sudden Holier-Than-Thou mood swings. If the players expect a similar kind of agency and want to stretch Lena's character as far away from her starting point as possible, then the game should track that character development.
This game either needs to track player's impact on Lena's personality with something other than "posh" variable or sacrifice agency by not letting Lena act outside of her character, or actually writing that behavior in a more plausible way so it fits all Lenas at the same time. Even a hidden "bad" stat would be appropriate in this sense, because it would help differentiate between Good Lena (never compromised her morality or redeemed herself) and Bad Lena (backstabbing horny slut who is not jealous anymore). And the funny thing is, there is such hidden variable which is relationship with Lola. But it's never checked in the game for anything when it could have indicated Lena's moral corruption the lower relationship with Lola dropped (she becomes careless of Lola's needs because her mind is in the gutter all the time).
All that said, while it might sound like I'm shitting on Eva's storytelling, I understand that the scenes are written first, and the plot is added later to fit those scenes, and not every scene acknowledges every possible way for the player to control Ian or Lena (plot-holes and inconsistencies become regular), and unfortunately the game is so big that you can't dedicate enough time to each storyline and consider each variation of Ian/Lena because the players want to get the story going and see more sexy scenes.
