There is so much speculation here that my brain is fried. But here's mine....
IF... IF.. Caylee is dead (not denial, just hope)...
This was Casey's direct doings and Cindy's indirect doings.
Casey couldn't have left Caylee alone to die in the car or the dogs would have hit on the car seat. I'm gonna speculate something happened in whichever house she was living in at the time, inside or outside, but I don't believe it was George and Cindy's home.
Gruesome enough, Pet Cemetery comes to mind. Bury your own, near you, or she may have wanted to frame Cindy by attempting to bury her in their own back yard. The dirt in the car could have come from Caylee's shoes. Not uncommon and reports do not tell us how much dirt. A stain? It was respondent to blacklight, which is indicative of blood. A stain which someone possibly tried to clean up. (Someone didn't watch enough TV).. She had to have taken this baby girl somewhere else after the attempted and failed burial at the Anthony home. The car starts smelling and she abandons it.
Cindy and George get the car and realize what the smell is. They know in their head what the stench is but their hearts can't process it. They track Casey down to find out where Caylee is. (Why is this important to Cindy at that very moment? Because that is her gut feeling). They call in a family meeting to get to the bottom of it. (From what I understand, Lee does NOT live in the same town). They place a bag of trash (bag of pizza) in the car in an attempt to conceal the odor, unsuccessfully. After all, she did call it a "bag of pizza" and it was her trash, no?
In a fit of desperation, they want to believe that Caylee is missing, nothing more, so they digest the missing story and call police. I can only imagine the conversations and questions going on in that household at that time.
Cindy is in denial, I honestly don't believe she had any physical part in this. George pulls police aside and admits they think Casey did something to Caylee. Point: He knew exactly what human decomposition smells like or he would have believed the missing child story.
From this point on, Cindy has put too much exaggeration into focusing on Caylee, not Casey. Clench jawed in court, reassuring Casey there is no proof, stick to your story.