I don't think Lena is being inconsistent -- her whole reluctance with Holly comes simply from her knowing that Holly has feelings for Ian, so she wants Ian either to commit to Holly or refrain from leading her on, in order not to hurt her. Lena making out with Louise or Ivy isn't going to bother Holly who doesn't have any known feelings for either of these two.
Yet at the same time, she can be also pursuing Holly as well. Let alone both Ian and Lena can be interested in Holly at the same time. Your not wrong in saying what you said. Yet to me, it adds more unneeded drama and complex things when it doesn't need to be fucking rocket science.
I made the suggestion ages ago, dismissed naturally by Maresy and others, that the whole Ian-Holly-Lena thing should be handled differently. Like for Ian, if he is into both of them, yet hasn't slept with Lena at the time (A hard choice I know), for him to admit his feelings to Holly yet mention he is interested in Lena as well, and of course Lena confess to Holly after the trip, that she is into Holly and Ian as well.
How is what I suggested entirely unphathomable? Just get their feelings out in the open and go from there. There isn't need for this pussyfooting around with maybes and longing for each other. Every thing can get sorted out once all the feeling are laid bare. But instead, we have Lena, if she is with Ian, still have the option to show interest in Holly and go about this 'Oh what would Ian say... I have feelings for Holly but I can't act on them...' For fuck sake... Just talk to Ian, tell him about them, then talk to Holly and go from there.