A lot of people assume that John knows about Nicole's and MC's affair, but what if that's not the case? Are there any hints besides whole "dinner situation" (kiss/gift)? The more I think about, the more I incline to believe this show was directed towards Silvia, rather than MC. John knows about Marcus situation and most likely heard conversation between MC and Silvia about waiting for the love or let feelings go. Also, John rubs kissing Nicole in only, when MC intervened. Otherwise nothing happens. As such, either John doesn't know anything or he still looks for opportunity to let MC know that he knows.
Regarding what happened between Part 11 and 12 - probably nothing sexual. In Part 12 I've only played route when MC doesn't go to dance, and went to Nicole instead. Considering her actions during this part (avoiding date, watching silly movies for teenager, hugs) I wouldn't be surprised if Nicole confronted John about his affair (she have seen John and Silvia kissing, then she found some panties under the bed) and he admitted that he is unfaithful for a while now