Them dropping the charges against April doesn't guarantee they'll stay dropped. Perhaps if she starts to get second thoughts about testifying then they'll start digging deeper to establish that she wasn't merely visiting. She probably left her GPS tracking on her cell phone, Aaron has already said she went to live with Nick/Kayla, she might have slipped and texted Nick about living there instead of using Signal; who knows!
After all, why were her credit cards in the master bedroom? Is that normal when you have a guest staying in your home that they leave their credit cards in the master bedroom?