Oops, I might have spread misinformation here. This is, in fact, not possible. I guess I was too blinded by the prospect of Ian getting a new cheating partner on ian_lena_couple route other than Cindy and Minerva. I forgot about the fact that I fudged ian_alison points to 6 to pass that check. At the time, I just figured I could have missed a point or two somewhere.Getting to creampie her in the latest update is gonna be the same scene regardless of which route you took to get to that point. I was just pointing out another path to get there that ffve didn't mention. Basically going on Alison's route, including the trip where she first shows you that low cut dress, but not committing to a love relationship with her there (because that blocks Lena route). After that trip, you can commit to Lena and become a couple. That leads to Ian breaking it off with Alison in chapter 10. Which should satisfy the condition to get her to let him cum inside in chapter 11.
So taking a deeper look, it seems that when Ian breaks it off with Alison over the phone, it sets their relationship to 3. He can apologize to bring it to 4. There are no more situations with her to raise the relationship score by the time she offers him to come to her place (after the Gillian thing). And the requirement for Ian to accept is 6 or more points.
So Alison's character is still trying her hardest to ensnare Ian, but Ian can't fall for it if he is on Lena route. Bummer.