Ok. It's weird because I have very similar playthroughs to yours. The only difference is that for my Holly path, Alison did sleep with Jeremy but only the first time. As a result I did get the conversation between Jeremy and Ian (where he mentions the threesome) on the first day of ch 9, which you didn't get. That seems like a bug.You don't have permission to view the spoiler content. Log in or register now.
As for blocking the more romantic Alison route, no I dont think that is a bug. I assume Holly is a legit love interest for Ian and if you chose to pursue a relationship with her in ch 8, it blocks all other future commitments from Ian.
As to why Cindy Alison works (for now at least) while Holly Alison does not, I think it's because Ian goes on the trip with Alison before meeting Cindy and telling her to break up with Wade. For Holly Alison path, Ian goes on the trip with Alison after he is committed to Holly, so by that point you cant select the heart options for Alison anymore.
At least that's how I see it, it can very well be an oversight by Eva Kiss.