What if...
June 15th-16th - Big fight, Casey takes Caylee and leaves. Caylee dies due to Casey's negligence.
June 16-18th - Flurry of calls, Casey gets through to GA, Casey tells him what happened, he tells her to bring Caylee back home and hide her temporarily and her will take care of it. He tells her not to tell anyone and pretend that nothing is different. Caylee is in Casey's trunk for 1-2 days, hence the decomp, hair and smell. Casey goes home and buries Caylee in a shallow grave under the playhouse. Casey washes off herself and maybe something of Caylees in the pool, which is then thrown back in the car, which is why the ladder is on the pool and why the chloroform is detected in the car.
June 18th-24th - GA unburies Caylee and sets her down in the yard while he finds someway to transport her. GA takes Caylee elsewhere and reburies her. He burns some evidence and gets rid of the gas cans. He reports the gas cans stolen and buys new ones to claim were found so that his are no longer "missing". This is why Casey can say that she doesn't know where Caylee is.
June 27th - Casey call GA and tells him that the car smells really bad. GA tells Casey to run down the gas and abandon the car with her purse, thinking it will be stolen. He tells her to use the out of gas story which is why he knows to bring gas cans to the towyard that day. Per GA's instructions, Casey abandons the car, leaving it near a dumpster either to mask the smell or to dispose of additional evidence in the dumpster. She leaves her purse in the car so that someone shady will steal the purse or the purse and the whole car. This way she can claim that someone else used the credit cards, caused the smell, and was responsible for Caylee's demise.
July 4th weekend - GA has landscaping work done in the backyard in an attempt to muddle the evidence. He moves the plant around, moves dirt around and puts in the pavers.
July 15th - GA picks up car from towyard. He throws the garbage out at towyard to pretend for the tow guy that the garbage must be the cause of the smell. He backs it into garage at home, and cleans it while Cindy is out searching for Casey. He notices the tan pants, backpack, etc and uses those details in his testimony regarding seeing them leave on June 16th to give it substance.
I think that GA has known all along and both he and Casey hid it from Cindy until after she had already called the cops and then they had to tell her because she was relentlessly drilling Casey and demanding answers. I think that LE should be, if they are not already, tracking GA's cell phone pings and movements from June 17th - July 15th.