Sure, the relationship fantasy wouldn't be complete if the cheating wasn't allowed (it leads to character development regarding their hearbreaks but they would have to face their cheating at some point). Robert is a non-issue because Lena ends things with him 100% of the time if she's seeing Ian. If she didn't end it with Mike in Ch 9, there's possibility that he might be involved in the cheating. It's just... both Axel, Mike, and Louise are fairly isolated from Ian's story (at least for now) and Ian realizing the cheating with them would only impact his relationship with Lena (if he finds out), but not affect any of his own paths related to other characters. It might make things slightly easier with Cindy because Axel would be preoccupied but that's it really. I guess Ian might become enemies with Axel but it's a ticking bomb anyway, things have been set up for him to become Ian's antagonist.
Jeremy on the other hand is a completely different beast and directly impacts lots of other paths for Ian. Alison, Emma and probably Ivy, that were all dependant on Ian's good relationship with Jeremy and Jeremy's status overall. You can't really avoid creating a massive bloat of branches by simply keeping Jeremy's sins a secret for a few chapters. It would simply be prolonging the inevitable. Those other paths wouldn't go anywhere. Eventually (in 3, 4, 5, 6 chapters) they all would have to be addressed, and can you imagine checking whether Ian is Lena's boyfriend, whether he's also been seeing Alison / Emma, been Jeremy's eskimo brother, their friendship level and all that? And there's no telling how far all of those branches would progress, there might even be more of them in the future based on Ian's interactions with Jeremy. In addition to all the other content that does not rely on Ian and Lena being together, or Jeremy fucking with Lena. You say that you realize the complexity of the task, and yet if Lena was allowed to cheat with Jeremy in a committed relationship with Ian, at some point entire chapter would need to be dedicated to Jeremy alone based on all the text that would result from that, and then people would complain that "Eva works slowly" / "there's not enough content because I don't even like Jeremy". And Eva probably understands this can of worms that Lena cheating on Ian with Jeremy would bring and can see it from miles away. So yeah, if she decided to save herself lots of time by not enabling it, I fully understand her and I'm actually glad that she spared me from even more work by doing that.
Or Lena could add her to her playtime with Ivy or expand her own redhead harem!
