I don't think she would have said the things in the last 911 call if she knew Caylee was for sure dead.
I venture to say that after placing that call, and the subsequent search of the backyard led them to finding the body.
See, things changed after that. The 9th became critical. They changed their demeanor. I would opine that Casey couldn't take the smell in the car anymore and buried the child in the backyard (borrowing of the shovel), perhaps to even set up her parents. The family found the body after the 911 calls and prior to the search by cadaver dogs. All the vehicles should have been checked, and hopefully all the pings are being followed. I think Lee knew first. They created a script and the big trip up to the date of disappearance being the 9th was the camera. Being former LE, George was certain they could track phone records and the dates had to be skewed somehow, someway.
When Casey says "YOU don't know what my involvement is?" and Cindy backing off like a scared puppy, it was the moment when they knew Casey might implicate the family. They then went on the mission to get her home and defend her from there.
jmo