Is the Ian/Holly route worth it? In my latest playthrough I got a little too close to Lena but tried to keep it casual yet in spite of that Lena was pissed when Ian slept with Holly on the book trip and ended their association. Now I feel bad and wanna go back and not sleep with Holly.
I'd only be interested in a Holly route if she goes through a bit of a transformation and becomes sluttier, more outgoing. If she's mostly the same as always then it would be best a friendship forms with her an Ian instead.
First, stay faithfully and don't play with Lena AND Holly. Lena is overprotective of Holly and loathes Ian if he play with her.
However, even then this can end in a threeway relationship later on, though it is currently still in the works. Also done by multiple ways.
If Ian dates Holly and Holly interacts with Lena and especially Ivy accordingly, Holly changes her hair, glasses, gives better blowjobs, sends nude etc. If Lena doesn't encourage Holly, she stays the same and has a innocent romance with Ian.
So yes, you can date a changed Holly. There is also the option to make her a slut. For that, Ian must not date her and he may get the opportunity to reconsider hsi stance towards her and date her much later, though this is speculative.