I know this is farfetched but what IF Casey is really telling the truth? What if Casey is not forthcoming because she believes Caylee or the family will be placed in peril if she tells all she knows? If Cindy and George even believe her a little bit they have to support her, to do anything else would be permanently burning their bridge not only with her but with any future visits with Caylee.
Like you, I don't see anything they've done knowingly to hamper the investigation. What they say to the media (to us) means nothing, it's what they tell the police that matters, and the police say the Anthonys are being very cooperative.