No way Casey will actually take the stand. Cross examination would leave the jury laughing at Casey's answers.
But, the defense strategy will be to stick to the ZFG story all the way because even if they can't prove ZFG exists, Law Enforcement can't prove she doesn't exist. To change stories now would be just too laughable and she is going to NEED her parents when this is over, so even if she hates Cindy with every fiber of her body, she will not turn on her. Casey has what I call "independence outbursts" but no clue to how to become independent. She's not going to make a ton of money with her "charms" and she won't like living that lifestyle long, anyway. She will go back to guilt-tripping her parents for shelter, food, clothes and spending money...if she doesn't end up with a life-sentence.