I think a couple things will be key in this case.
a. the duct tape
b. the decomp in the car
I really think Caylee was accidentally killed by Casey, and Casey wanted to have that car stolen, with Caylee in the trunk, in the garbage bag, with duct tape over the mouth.
She intentionally left her in the trunk, and I think she hoped her car would get stolen. That is when she would call the police and be the distraught mother whose car was carjacked with her baby in the carseat. The police would find the car, and Caylee's body would be in the trunk.
The problem was that the car wasn't stolen, and the decomp smell started. Casey had to come up with plan B, slightly different, but the car was left in the Amscott for a reason. And gas has nothing to do with it. I think she again wanted that car to be stolen.
The disposal of Caylee in the woods was not Casey's first choice. The hysterical mother whose kid was kidnapped from her would have been a better option (giving Casey all of the attention), and I really think that was Casey's primary intention. The Zanny story came up later, as a way to bridge the lies together when the original carjack/kidnap plot was thwarted with no one stealing the car.
I still say Caylee was in that carseat after she died. With duct tape on her mouth. It reeks of a Susan Smith scenario gone wrong.