This is nuts. The REAL absolute biggest plot hole.
With the injuries the pedophile had inflicted on Margot, a case a justifiable homicide was in the bag.
In the USA, she'd have been questioned, released, and patted on the back in short order under whatever circumstances. Women aren't expected to just be beat senseless by criminals.
Even in France, with a father that is police brass, the situation could have be made very presentable with some thought and preparation.
Its a good movie, but without a reason to exist.