I voted that Casey killed Caylee and that her whole family knows. I think that grandpa knew first, then helped Casey by cleaning the trunk as best he could and was satisfied he had done a thorough enough job to prevent the odor from being detected.
Afterwards, he directed Casey where to park the car and told her to leave her purse on the seat to entice theft. He would know that LE would find other prints on the car door handle even if no one had stolen the car, but had broken in to steal the purse. Only this did not happen and he had to deal with the car the second time when it was towed and he picked it up. I think that Cindy knew of Caylees death just after grandpa did and a family coverup was set in motion. And yes, I do think that Lee knows as well.
I also firmly believe that Cindy knew of Caylee's death when she wrote her MySpace message on July 3, 2008, that has since been removed but a copy remains on this board somewhere.