Yes, and any such possibility was a bug which was patched out. Lena in relationship with Ian simply holds different opinion from you on this matter and won't just casually fuck Holly. You may consider this not to make sense, but then it's simply a difference of opinions between you and the game's author, on what the character is like.
That's quite incorrect. Holly even on her "slut route" is attracted to Ian. In fact, she's more readily showing him this attraction on this path, actively flirting with him.
I never said she wasn't attracted to him. But I think her attraction to him now, is for different reasons. She's pursuing him to prove to herself, that she can be the equal of Lena. Because Ivy has built up her confidence to believe she is. The question is, if she finally gets him to sleep with her, will she stay attracted to him or move on to her next conquest? At this point, she pretty much does whatever Ivy tells her to, and Ivy is probably telling her to not catch feelings for anyone, and just play the field.
What you claim about Holly is contradicted in the current text, where Holly expresses bemusement at Lena not having sex with her and Ivy during the shower scene. If Holly isn't worried at all about Lena having sex with her, or whether that constitutes cheating, why would Lena be worried about protecting her feelings? That makes no sense.
And it's quite obvious, that the content was intended to happen previously, but the Dev for reasons known only to herself has now decided to omit it. The threesome with Louise and Jeremy was linked to the threesome with Louise and Mike on the cuck route. And because Jeremy content has now been omitted, if Ian and Lena are in a relationship, that cuck scene which makes perfect sense if she's already cheating with Mike, is omitted as well.
At the time, I believed initially that the threesome with Louise and Jeremy turning up on the Ian/Lena relationship route was a bug, because Lena says nothing about cheating on Ian during the content. But in a later update, that scene was directly linked to another threesome involving Lena, Mike and Louise on the cuck route, which suggests the Dev did initially intend for this earlier threesome to be available on Ian and Lena's relationship route, because why else would she link it in the dialogue to this later scene?