I think that's exactly what happened..............
I think there's a good possibility that whatever happened to Caylee, happened somewhere other than the grandparent's house. I think Casey stashed the body somewhere for two days and then put it in the trunk of her car and proceeded to the grandparent's home with the intention of burying the body there.
I think Casey backed her car into the driveway because she had the body in the trunk. She borrowed the shovel and waited for the neighbor to go into his house.
Then she removed the body, proceeded to the backyard, and laid the body close to where she started digging. She found the digging hard and moved to another location in the yard, moving the body closer to that area. Again, she found the digging hard or an obstacle like tree roots, and moved to yet another location, moving the body again. After about 45 minutes or so, she gave up, returned the shovel, and put the body back into the trunk. This accounts for the cadaver dogs hitting on three locations in the backyard.
When she found she couldn't bury Caylee in her parent's backyard, Casey had to consider other options. She may have left the body in her trunk overnight while she considered other locations.
I think she may have actually disposed of the body on the 19th.
There's no way George opened the trunk of Casey's car on June 24th and found everything "normal" with the exception of finding his missing gas cans.