I'm as clueless as you are in this regard. I only know information up to episode 3, re-reading some manga chapters, and playing the trial version (which I can't read, so the most I know is that some scenes were either adjusted, omitted, or switched in order).
From the manga alone some specific scenes were more accurate to the game (like meeting Satsuki while going to school).
It's implied the hatred for chocolate is from something from the past, clearly. But that's all I know. I don't know any specifics beyond that and would like to know too.
I personally don't think that's connected to the Oosawa Incident. If it's that incident I would find it more relevant to Isara because she's a financial aid student (which is specifically what the complaint was regarding).
But yeah all I can say is maybe related to the past. Episode 4 might help but the focus is on funding, not the past, so I don't think any more information would show up there.
I was watching the KoiChoco anime right now (and re-reading some manga chapters) and Chisato's biggest hatred for chocolate is the biggest question I've been wondering for some days. I don't play the game, I just want to ask why she hates chocolate in the first place : is it connected to the Oosawa incident or not ?