I agree -- the coddling support they've been giving Casey hasn't gotten them anywhere. Time for something different. Maybe if Casey believes she will lose her entire family forever -- that there is no one who believes or supports her -- that she just might eventually crack and give up at least a tiny morsel of truth in order to hang on to her family. Until she does, though, I wish the family would not communicate with her whatsoever.
Casey was caught off-guard the day her mom found her and she hastily made up the ridiculous story of the nanny, etc., without thinking it through. She may have been thinking all along what she could tell people happened, but she hadn't put it all together at the point Cindy surprised her...she probably thought she had more time to come up with something or maybe she just plain couldn't think of anything plausible.
That her family stands steadfastly by her and insists she's telling the truth -- in the face of so many proven lies Casey's told about this -- says to me that they are looking for ways to excuse her, as they're accustomed to doing. I have no doubt how much the GPs love Caylee and how heartbroken they are, but they need to stand steadfastly by their granddaughter, without regard to the consequences for Casey.
IMO, Casey's probably spending her time in jail trying to concoct a more realistic, plausible scenario. She and her attorney are waiting to see what the DNA testing and other evidence shows, to see whether LE has enough to charge her, and then and only then will she reveal a newly fabricated story...one she hopes will get her off the hook or at least lessen the consequences.
Should the evidence stack up against her, she might share some of the real truth in order to try to strike a deal for herself. Caylee's whereabouts and the details of her disappearance are Casey's bargaining chip. She's not gonna give that up because she may need it to help herself if/when charged. In light of that, I'm not sure any approach the parents take will be helpful, but cutting her off completely is worth a try, at least.