Oh god. It just hit me. This could explain it, but it's only my wacky idea.
Casey died on the 16th, possibly at the house. Of the calls, none go to the house during the 4-4:30 "flurry." So possibly something happened to Caylee between 1:30 and 4:00. Caylee's body may have been at the house on the 16th, overnight until the 17th.
So, let's say Casey killed Caylee at the house and then hid Caylee's body at the house or in the yard on the 16th.
On the 17th...she borrows the shovel and buries Caylee in the back yard.
Then, overnight, worry that someone might find her.
On the 18th, backs into the driveway, goes to the back yard and pulls Caylee out of the ground...it's easier to pull a body from dirt that's been replaced in a hole than to dig a hole. Takes her to the trunk and then to another place to dump her.
This would explain the dirt in the trunk, and other details.
I'm sure that there are 100 other explanations that are just as logical...this just hit me, though...quite heavily.