Based upon this "standard" you propose here, if the police raid the home of a suspected serial killer and unearth the skeletal remains of a couple victims in his yard, they should not charge him with murder, nor should a jury convict.
After all, they don't know exactly how he killed them unless the skeleton shows trauma. They don't even know for an absolute fact that he killed them at all. It doesn't matter how many skeletons there are. One, two, twenty-two, it could be a coincidence. Perhaps they all drowned, right? Even if our maniac was the last person seen with every one of them, still not "proof." He is, at worst, only guilty of undertaking without a license perhaps, or improper disposal.
In this case though, we know that Casey made statements indicating that she KNEW Caylee would be out of the way. We know that she was not only responsible for her daughter, she was the last person seen with her. We know that she lied to conceal the kids whereabouts from friends and family. We have every reason to believe that there was a dead body in the car trunk. We know that the cause of death was a rare duct tape that Casey had access to. We know the body was found in a special laundry bag which Casey was only able to secure from grandma's home. We know that Casey lied about her job, boyfriends, babysitter, location, and pretty much everything else. We know that she attempted to sidetrack, delay, and derail police investigation into this murder. We know that Casey got a tat celebrating her new wonderful post Caylee life. We know that she spent the 31 days before her child was discovered missing partying hard. We know that when the car was found it reaked like a dead body.
These are all facts and none of them have been seriously challenged. Nor do any of defense theories amount to anything more than "maybe" and "possibly" and "theoretically, it's vaguely possible."
The prosecution offered a solid motive, they showed how Caylee died, they showed that the only person who could have possibly done the crime was Casey -- she was the only person with access to all the pieces. We know all this. Was there a reasonable doubt? Were reasonable questions left unanswered?
Not if you review the evidence in the case.