That is a possible scenario. It was in fact addressed by the prosecution. Even if it happened that way it was still 1st degree murder.
"If she was charged with manslaughter she would have been found guilty as charged."
She was. It was one of three possible charges the jury could have chosen.
"By the time the body was found in the swamp, it had decomposed too much to detect a cause of death."
A cause of death is not required to prove murder.
mlo: She was. It was one of three possible charges the jury could have chosen.
Actually the option was aggravated manslaughter, which did not fit the manslaughter scenario presented to the jury.
It was a slick sabotage of the court process. The prosecution made sure that the charges - both murder and manslaughter - were too extreme for the evidence. Involuntary manslaughter would have nailed her, and possibly murder two. But that's not what they charged her with.