If anything the slap and subsequent police report bolster the idea that he was abusive, both physically and emotionally and that she was very much the victim.
I agree.
He reported a slap (possibly spuriously) as a way of saying "I can lose you your job; I can destroy your public reputation; if you try to leave me I will be the one awarded sole custody of our child - you'll be lucky to get supervised access.
Know your place woman."
And when he'd done that, he tried to withdraw the complaint (because let's face it - it made him look an *rse, didn't it?) but the police have to act on a complaint of domestic violence no matter how ridiculous it might seem, and so there is a record of it. A record that should never have seen the light of day, IMO, because it was irrelevant to this butchery of a helpless woman and child.
So a woman who was either heavily pregnant or who had recently given birth was dragged into the police station during the night and made to give an account of the incident.
I wonder if the police were more worried about his behaviour and were trying to make sure that Emma was safe - and if she was too frightened and intimidated by him, too worried about her job and her baby, to speak against him.
Or maybe she really did blame herself. These gaslighting men, as @DorritLittle said earlier, really do make their wives and partners feel that they are literally losing their minds and don't even know which way is up.