Wow, the pool theory seems VERY plausible to me. I've been struggling with wondering why, if Caylee was killed somewhere other than the grandparents' house, would the decomp smell be in the yard? It would make no sense to bring her there if she was already dead. If Caylee drowned, Casey could have stashed her in or near the playhouse, possibly even for a few days while she figured out what to do. Finally backs the car up to the garage, puts Caylee's body in the trunk (with some dirt clinging to clothing, perhaps), and takes her ... where??? I tend to think that she ditched the car immediately after she disposed of the body - maybe she didn't realize the car would smell so bad even after a short while.
I don't know much about decomp - how long would a body have to remain somewhere for the smell to linger? Also - do we know of anything near to where her car was left? Lakes, woods, fields, etc ... ?
After all these events, Casey is so freaked out she can't stand the thought of being around her parents and them discovering what happened. Hence, she stays with friends and Cindy doesn't know where she is for a month. During this month, she is developing the kidnapping story and convincing herself (as pathological liars do) that it is actually true.