Just so people don't come away with nothing but watching a lesbian pedophile and partial nudity, here's my interpretation.
Cracks is about the psychological crack of Miss G. Miss G is someone who has been caged all her life, being a former student of the institution. She's full of passion and desire (remember how she believes desire is the most important thing), but she was never able to fulfill any of her dreams. She got by just pretending and playing a role until Fiamma came along. To Ms. G, Fiamma represented everything she desired: adventure, the exotic, romance, and travel. Fiamma is everything Ms. G wanted to be, and so Fiamma reminded Ms. G of how reality was starkly different from what she desires for herself. That's the trigger to what makes Ms. G crack. Then, add the fact that Ms. G never got married. Remember how uncomfortable she was when she went to the pastry shop? It's because women were suppose to be accompanied by a man and Ms. G never got married. This isn't a movie about a pedophile. It's more like a fanatic that ends up killing their idol. It's about the kind of envious lust that fuels a dangerous adoration. The limits of desire and the loss of control.
I recommended watching Leaves of Grass starring Edward Norton because there's a nice parallel in themes. In the beginning of the movie, his character talks about the illusion of control and its breaking point.
Some very good insights here @MonkeyKing, but when I watched the movie (haven't read the book), and the scene where she has a sort of mental breakdown when she goes to buy pastries - I thought that was some sort of social anxiety manifesting itself? I mean, sure, in the confines of the school, she is confident, adored, and totally in control, but once she leaves her 'comfort zone' which is the school, and goes out into the outside world where she has lost all of her perceived 'control', she is suddenly an insecure, mumbling, confused & paranoid wreck.
The persona she has so carefully created for herself within those walls and paraded in front of the young girls is nothing but complete deceptions & lies - remember when she's on the dock, and the guy on the boat asks her if she's going to be traveling on the boat, and she says something like 'no, I can't stand open water'. She's never travelled, never lived any kind of adventure or life outside of that school. The real outside world scares her.
Mrs. G begins having a psychotic mental break - I think due to her sense of loss of identity and control - when Fiamma begins exposing her lies in front of her devoted flock, deeply humiliating her & threatening her carefully constructed world.
I also thought the rape scene in the movie is not so much about Mrs. G lusting after/desiring Fiamma (although she obviously desires Fiamma's more fulfilled life), but more of a desperate need for her to finally exert some kind of power & control (+humiliation/punishment) over the strongly defiant young girl, which up to that point she has frustratingly been unable to do.
*But*, again, I haven't read the book, so I could be totally wrong.
------------------------------------------------------------------------------ If you could have any superpower, what would it be? -The power to make you interesting.
I tend to mostly agree with your points, Miss G was certainly Agoraphobic to some extent outside the confines of the School, Institutionalised also (the 5 objects on her bedside table at the end reinforce this). If one is to fill in the blanks and what little we have to go on it's possible to jump to numerous possible conclusions. I'd guess she never once left the Island since joining the School as a pupil, maybe she was orphaned which added to her agoraphobia, maybe she herself was abused in some way (sad fact but many abusers were victims of abuse themselves) perhaps even by Miss Nieven (Sinéad Cusack) or perhaps not but there was certainly a lot of history between them, there are numerous other possibilities of course.
I do however think she was a pedo, the late night skinny dipping so she could see Fiamma naked set the certainty ball in motion, the actual molestation of Fiamma set that fact in stone, even if she hadn't done that sort of thing before her earlier treatment of Di was certainly a bit iffy.
Not a bad film, well acted and well shot but the story seemed to swim around in circles for a while about half way through until the midnight feast.
Question... What medicine do you think Miss G gave Fiamma when she forced her to dive again? Was Miss G also perhaps an addict of some kind of Opium based prescription medicine? She certainly started to look a lot more like an addict once the Cracks in her facade were revealed and she started to go a bit more bonkers.