I agree with your entire post EXCEPT how Caylee died. Based on the internet searches and having some insight into Casey's lifestyle, need for independence but no job, saddled with small child, young, strife with parents, especially mother, etc. etc, I think Casey thought about killing Caylee for a long time. I think she planned her way around it for a while but never got the courage to go through with it. What was the trigger was the fight with her mother. I am sure Casey's thoughts went along the lines of, "She won't ever see her again" everytime there was an argument. But the thing is, Casey needed her mother. After the last blowup, the best way to 'get back' at her was to get rid of Caylee. It would solve all her needs - the freedom, the partying, and best of all, what better way to break her mother's heart. I really believe there is a strong psychological component to this that goes back many years. I don't think this was an accident (she would have reacted differently) or a spur or the moment thing that suddenly popped in her head. I think she did act in rage - don't get me wrong - but the seed was planted a while back. Just my opinion.
Rest in peace Caylee. I thank you every day for giving me more reason to love and appreciate my little girl.