Is there a way to have a loving IanxLena relationship, where they're dating and all, but agree to an open relationship? I had tried it but always failed. In one play Ian fucked Holly while visiting the book festival and when Lena found out, she broke up with him and Holly did so as well, basically he lost them both. In another, I had Lena seduce Jeremy and then she refused Ian to become his girlfriend, although they were still FWB. I know there's a cuckold path for Ian as well but I'm not interested in that branch, it just makes no sense for him to remain in a sexless relationship as a complete wimp while he dominates his boss and Alison.
In my opinion they work best as a loving couple, but with lax boundaries. Like a stag-vixen kind of relationship. Are we even that far story-wise? Will this be an option at all?
No.
The open relationship variable does nearly nothing as you can see, only a few dialogue switches - and allows them to discuss the later threesome if Lena has low lust.
I understand what you're getting at with your overall post, and the answer is again no. I'd very much like a path like this myself, and as someone who's spent hours browsing and modding the scripts, it's quite clear evakass has no interest in something like this, and far prefers the bullying/cheating themes.
This is extremely evident with how the "ian_cuck" path flows; it's binary, once you're on it, you can get off it by answering "nah I dont wanna hear those stories of your professor" during the footjob scene (iirc there are some other places to get off it), point is, it's either sexless ian with humiliation, or full relationship with fucking, and Lena keeps any indiscretions secret. Again, it's crystal clear what eva likes to anyone who's read the code at length.
Don't listen to the others trying to offer half-cocked story-based refutations of this. The reality is some code segments just aren't that developed (which is understandable, only so much time in the day for dev). Another example of this is the part with Lena-Ian relationship start you mentioned. The Lena-Ian relationship fork is conditioned on NOT v8_jeremy_sex, yet then under that condition branch, there are additional checks for v8_jeremy_sex, so it's not clear from the sloppy coding if that was supposed to block the whole fork or not.
Having said all that, Alison has a few variables that look to leave future options for stuff like that open, unfortunately, it looks like it'll just be with Jeremy again - I'd expect a future 2nd threesome scene with them in a future update.