I'm just going over scenarios in my head that might make any sort of sense. Perhaps the grandparents went on vacation, Casey is at their house with Caylee, they are playing in the backyard, Casey falls asleep/ gets distracted with the pool ladder up, and Caylee goes exploring. Casey then discovers what she's done, loses it, because she blames herself (rightfully so) is scared of her mother, and just panics. Thinks about what to do for a few days, while her parents are on vacation, leaves the body in the backyard hidden maybe in the playhouse (that would account for the dog hits), right before her parents are coming home, she decides she needs to move Caylee, and invent a story.
In my own head, i just can't see her doing something to her daughter intentionally, they just looked too happy in pictures.
As far as the grandmother, i feel that she didn't know any of this, and I don't feel like she is so much protecting Casey as she is herself. She is completely in denial IMO and doesn't want for a minute to believe her precious granddaughter is dead, so she will look at any crazy thought or explanation to rationalize it in her head because the alternative is too much for her to deal with. Deep down she may know, but she is trying to hang on and keep going because i think if she really sits down and thinks about it, she's going to realize what's happened and it will be too much for her to bear. She is projecting her anger for her daughter on to anyone else who is daring to make sense of this. She isn't guilty of harming her granddaughter i don't believe