Okay here is my theory - it is very simple:
Casey and mom fight over money, responsibility and custody of Caylee in early June, by June 9 they made the split from each other official and Cindy gives tough love, Casey leaves with Caylee (this is why this date was said by both, it was a pivotal date in both their minds).
From June 9th to June 15th - KC has been leaving Caylee asleep in the car, by giving her chloroform to keep her asleep, she checks on her from time to time, while her BF is at school she feeds her. By June 15th Caylee was back in CA's house for father's day only, then on June 16th she is back in KC's care, KC had no plan of action. The flurry of calls on June 16th are attempts to get them to take Caylee back so she can do her thing, she needs them to baby-sit again, she needs money, but they ignore her calls due to tough love.
KC with no plan of action, she wasn't working, she had no place to live, no money and she had a new man who thought she was the bomb. She steals money to support her fake image. She is getting overwhelmed and gives Caylee a fatal dose of chloroform so she can go out and leave her sleeping, Caylee dies either by accident or on purpose, she places her in the trunk and tries her best to forget about her until she figures things out.
In late June or July she has to dispose of the body due to the close call with GA and the gas cans she stole to FUEL her car. She drives a long distance and dumps her in a body of water no where near her home, on her drive back she runs out of gas and leaves the car at the AMSCOt, she knows the car stinks and she collects garbage from the nearby dumpster and leaves it in the trunk to mask the smell.
The rest of the time between late June early July she is spent lying to everyone and making mistake after revealing mistake. She is so bold and arrogant that she thinks she can play this out for a while until she devises a new story and plan.
When Cindy finds her - she is prepared with a story, but not prepared to not be believed.
and here we are today.