I just made a playthrough like that.
Ian fooled around with Cherry, Alison (+3some), Minerva, Emma, Cindy (didn't go on trip with Holly), flirted with Ivy and Louise (+3some Ian/Lena/Louise), and was dominant with Lena. Lena fooled around with Robert, Mike, Jeremy, two 3some with Louise, Seymour, Ivy, Stan, Marcel, Mark, Jack, and teased Perry and Emma. The two MCs were in love with each other (and didn't love any other LI) but were not in a serious relationship after the park scene (because Lena fooled around with Jeremy), just dating.
And I still managed to get the good chapter 12 ending (good relationship between Cindy and Wade despite the breakup, and Perry scored with Emma at the beach house)
There is two requirements to have the throuple open in this path:
1-Lena don't desire Axel. Lena can pose with him in the Seymour's photoshoot, call him for another photoshoot but she have to reject him when Axel tried to have sex with her at the photoshoot and later at the club. To make the rejection easier, when Lena had a dream about Axel, she have to stop it. If this requirement is fulfilled, you can also access to the pure love scene between Lena and Ian (become a couple late) if you don't chose the throuple (or the breakup, of course).
2-Holly musn't be under the influence of Ivy. Lena don't incite her to see men and go home with Holly instead of staying at Ivy's place.