My updated theory:
Casey is a sociopath and has Borderline Personality Disorder. The family has known for years that Casey is mentally ill and incapable of taking care of herself. Tensions between Cindy and Casey escalated and Casey was thrown out of the house or left on her own accord. There was no "vacation" as Cindy alludes to. Casey stayed with various friends including the boyfriend. Caylee was left in the trunk for some time decomposing. I believe at one time Casey may have buried or tried to bury her in the parents backyard but moved her. Both dogs targeted the backyard. The neighbor saw Casey BACK the car up to the garage, very suspicious, especially since she was no longer living there. The parent's shed was broken into and gas cans stolen. Casey borrowed a shovel. HELLO? All during the time her daughter was supposedly kidnapped and Casey was searching for Zenida in bars. Casey may have had something to do with the robbery of 'A' Zenida Gonzales or knew who did or picked this name from the story ran in the papers and internet afterwards. I would be curious if this SAME Zenida was the one that inquired about the apartment rental.
My prediction is that within 2 weeks, the evidence against Casey will be processed and she will be charged with murder. They may choose to conduct a search for the body at that time, but I have a feeling Casey will fess up and state it was an accident. So there you have it. Plain and simple. Just another mentally disturbed human being in a dysfunctional family that may have acted in a rage against her mother or simply was not mentally equipped with raising a child on her own and may even have left her or locked her in that trunk of the car while she went out to party or killed her daugher and hid her ther for some time afterwards. There are NO kidnappers; NO drug deals gone bad, NO mafia, etc. Just a troubled young woman who has already exerted signs of mental illness and a family (no mother) who is in deep denial. I believe the father as stated knew his daughter has something to do with this crime.