Have played through with different Lena/Holly variations now.
First was non-pole-dancing (and therefore non-Ivy) Holly. Just now I played through emphasising Holly's change variable but keeping the romance intact (not doing the first threesome with Ivy), I did different saves trying out both having Holly not do Mark and do him and including Ivy or not.
You get the option to do the shower scene with Ivy and Holly if Ivy and Lena got busy after the Axel confrontation at the Ivy birthday chapter. You can use a willpower point to not give in to Ivy or to protect Holly after giving in to her, probably meaning that if you do give in and get the shower-threesome scene you might lose the Holly romance path? Willpower use = Lena and Holly staying romantic and ending the Ivy threesome path.
Curiously Mark does not feature in any flavour dialogue when it comes to Holly and Lena's romance. He only really comes up in the dialogue specifically about him fucking Holly, but the Ivy shower scene changes the tone after Lena and Holly's later sex scene. If you do the shower scene then Lena will not express a deepening of her and Holly's relationship. If you keep Ivy out of Holly sex scenes, Lena will narrate those thoughts regardless of Marks involvement with Holly too, or how much he got to share of her (both doggy-style photo and the BJ scene where Holly looks into the cam if you told her to release her inner slut).
I suppose I get why Eva hasn't gotten around to giving Lena/Holly players the option to officially have them be a couple yet, but I wonder how much further it will be given how both Mark and Louise seem to have unclear roles for that route still.