Here is what I think could have happened, ..but since its speculation don't hold me to factual reference
Assume the drowning story, then:
*Assume there is an element to the drowning which could point to culpable negligence
*GA does in fact tell Casey "you'll go to jail for the rest of your life for child neglect"
*Casey believes it (not hard to fathom).
*GA feels he too will be blamed for this Cindy/LE/media or that the death would prompt discovery of something else, for whatever reason GA decides that its in his best interest to cover-up and Casey is afraid to go to jail for the rest of her life.
*GA agrees to keep quiet, tells Casey its her responsibility to properly bury Caylee
*Casey buries Caylee in the back yard, tells GA when asked what she did
*GA becomes furious tells her to get Caylee's body off the property
*Casey removes Caylee tells GA she will go bury her somewhere else
*Casey gets cold feet - leaves body in trunk, escapes (mentally)
*Car starts to smell - Casey puts Caylee where she is found
*Casey tells GA that she buried Caylee in a cemetary and marked the spot
*GA goes there and cannot find the spot, starts to suspect Casey just dumped the body
fast forward to December 2008.
*By this point State had ALREADY (before body found) put the death penalty on the table, in an effort to get Casey to point them to the body
*Casey has told JB the story - he calculates she'll never get convicted of murder 1
*After body found w/duct tape, State will accept plea for manslaughter, nothing less (regardless of what story she tells)
*
JB/Casey decide to roll the dice in court vs plea to manslaughter
*Casey sits in jail for another 2.5 years in an effort to get off with time served (checks, lying to LE which JB tells her will be 3 years anyhow) rather than accept 10 year sentence in a plea deal on top of that.