I'm more apt to believe that she borrowed the shovel with the intent to dig an adequate sized burial hole in her parents back yard (or somewhere), but when the dense little demento realized either:
1) Digging a hole is a lot of hard work - particularly in a lot of central Florida soils.
2) There is always a noticeable mound of dirt when you bury something, unless you can find a place to put the displaced dirt (replace by the object/body) that no one would notice.
She gave up with they burial idea, put Casey back in her car truck and PROBABLY easily tossed her into any public dumpster sometime later.
Since weeks went by before Cindy hunted Casey down for answers about Caylee, Caylee's body could be in a landfill under tons of garbage wrapped in an inconspicuous garbage bag.
Talk about a needle in a haystack...
For Caylee's corpse to have rendered Casey's car unusable with suspicious stench, she would have to have been in the hot car for no more than a day or two within her time of death. By all accounts, that's how quickly in hot air decomposition occurs, and the odor can not be removed.
I think Casey abandoned her car with her purse inside in front of a check cashing store to attract a car thief, and avoid any of her friends or family catching whiff of it's interior while she was driving it. She knew by the time the car was abandoned that she would need a plan. It was one of the first steps in developing a kidnapping plan.
sounds good