I am sure somebody else has mentioned this already, but the fact that Nick is going after Nanny Chaney and not "Nanny" April really reinforces in my mind that he and April aren't completely separated.
Nick does not take rejection well, and April would be a far better candidate to try and blame if he's trying to get ahead of the bodycam release. She was there right up to the arrest, even though we know the whole Nanny thing with her was a cover.
Instead, he has to go back to the Nanny who quit in February, three months before the arrest, to try and find a scapegoat. It's really fucking weak, but it's about all he's got.