I don't know if this theory has been discussed, but the ladder in the pool really disturbs me. Cindy claimed they put the ladder away, but when she returned home, it was in the pool.
Here's what I think possibly happened: Casey and Caylee decided to take a swim while the rest of the family was gone. Maybe Casey got a phone call or had to run in the house and left little Caylee in the pool. In the time it took Casey to get the call or do whatever she was doing, maybe Caylee accidentally drowned in the pool, like you hear about the parent's that leave their tots in the tub to answer the phone and when they come back, the baby's dead.
So Casey freaks and pulls Caylee out of the pool and doesn't know what to do. It would be interesting to know if the cadaver dogs hit near the pool... Instead of calling paramedics, like a normal person would do, Casey is afraid of getting in trouble in her freaked out state, so she calls the only people she thinks she can trust. The flurry of phone calls begins as Casey tries to figure out what to do. Maybe she brought Caylee in the house, which would explain the (unconfirmed?) reports about the dogs hitting in the house as well.
Since no one is apparently answering thier phones, Casey, still freaking out, puts Caylee in the trunk of the car and drives around, plotting out her next move. Maybe she gets help from someone she knows??? Casey borrows the shovel from the neighbor and she/they take care of disposing of Caylee's remains. No clue what the gas cans were used for, nor do I even want to speculate on that aspect of this.
To try and get rid of the evidence, Casey orders a pizza, puts it in the trunk and dumps the car. Maybe she thought it would be towed away and forgotten. I can't see Casey walking, I'm willing to bet she called someone to come pick her up right after she dumped the car. That person could possibly be her boyfriend???
I know this is chilling and it isn't meant to be taken as fact, I'm just trying to explore a plausible theory as to what happened to little Caylee.