I had a theory before I read this thread, but while I was reading it, y'all brought up evidence that I didn't know about before and I changed my mind
Initially I thought that Caylee died by "accidental" death by trunk over the night of the 16th, when she was palling around with her new boyfriend Tony, who didn't want Caylee around. Casey wouldn't have taken the plea agreement, even though it was an "accident", because then she'd have to admit that she was a horrible mother who stashed her kid in the trunk when it was convenient. She might have gotten away with this all winter and maybe didn't realize quite how hot it was on the night of the 16th. I know here in California, when the temperature is 100 F during the day, and it drops to 78 F at night, we all say, "Thank goodness it's cooling off," even though 78 F really isn't "cool" at all.
However, then I learned that Casey didn't pack any extra clothes for Caylee before she left on the 16th. It seems to me that the most pleasure that Casey got from Caylee was using her as a big ol' Barbie doll, so not bringing the clothes was extremely out of character. Casey had either already killed Caylee or was planning to kill Caylee when she left the house on the 16th, so it was pointless for her to bring clothes for a corpse/future corpse. Now I don't think it was any kind of "accident" at all
I really don't think Cindi would have called the police at all if she knew Caylee was dead at Casey's hands. I think Cindi had bought into the whole nanny story and needed police intervention to get Caylee back. I can believe that calling the police was intended to force Casey to tell them where Caylee was. I think George had his suspicions but kept them to himself in order to avoid more tongue-lashings from Cindi. That's why I think the cover-up was so poorly executed: if George and Cindi had played a significant part in it, Caylee's body would never have been found.
Now I think that Casey simply smothered Caylee during the day on the 16th. I'm not sure if she did it in the house or not. I don't really believe George's revised "hugs and kisses" testimony that he gave on Nancy Grace---it's too "Leave It To Beaver" and sounds too out of character for George. After Casey smothered the girl (might have been done with the tape) Casey took Caylee out to the trunk and forgot about her for a few days. Finally she gets around to dealing with the little problem that's starting to stink up the car, and goes to the Anthony house to do it. She backs in and stashes Caylee's body in the playhouse for a while and looks around for a shovel, finally borrowing one from the neighbors. Obviously burying her was too difficult, I mean, that dirt is really hard and Casey might chip a nail or something! Casey switches to Plan B. She moves the body poolside, gets the ladder, and tosses the girl in. While poor Caylee is floating, Casey has second thoughts about Plan B. I'm almost positive that the whole ladder issue was something that Cindi lectured and nagged about continually, so maybe Casey envisioned this nagging, amplified and never-ending, for the rest of her life, and changed her mind. Maybe she thought back on all the episodes of CSI she watched and realized that the autopsy would show that Caylee didn't have any water in her lungs and died before she went in the water. She rejects Plan B. By this time she's a little panicky, since her plans aren't working out, so she forgets about the ladder and the gate when she drags Caylee back out of the pool. Caylee goes back in the trunk for a few days while Casey tries to think up a fool-proof Plan C. This Plan C never materializes, and by the 19th, Caylee is really starting to smell bad, so Casey ditches her in the woods near her house, which really isn't the most well-thought-out plan. She purposefully abandons the car, hoping she can claim later that someone stole it, in hopes of explaining the stench, but before that can happen, the tow-yard gets it.
I think George knew that Caylee died in the car the minute he smelled it, but he held his tongue. I figure he thought that what was done, was done, and there wasn't much he could do to protect Casey other than clean up the car. Cindi was just in denial about the whole thing until some time after she'd already called the police.
Anyway, that's my new, revised theory. If any of you can pick a hole in it here or there or give me more information that I don't know about, I'd appreciate it. Wouldn't hurt my feelings a bit if you did. Y'all seem a lot more knowledgeable about all of the evidence than I am.