I think it was to have all started out as an abduction from the park, but something went wrong, she chickened out, and the odor got into the vehicle. If the odor hadn't gotten into the vehicle, Casey could have pulled an abduction after the first week and made up a different babysitter. After the odor was apparent she knew she was screwed because police would check it out. Then, she was probably going to burn the car, especially the trunk with the gasoline, but knew that wouldn't work either, because Caylee hadn't been seen in a couple of weeks, and she could produce no nanny. She didn't have enough money to really go anywhere, and Cindy was threatening to turn the car in as stolen. After the car started smelling she knew she couldn't claim adbuction, and after 31 days she knew the nanny story wouldn't work either. She was cornered, and the only hope was to convince Cindy to NOT call cops just yet. That was her only hope, but it didn't work either, so she told Cindy the 2 year old successful Zanni lie, and had to keep it up with the cops. If it weren't for the smell she could have said she'd allowed a nice lady she'd befriended at the park to watch Caylee for a few nights because she was homeless, and admitted all the lies about working.
Casey told the cops herself that her biggest mistake was not calling them first thing, and she's right. Also, someone did look up missing kids on the computer.