I don't believe Casey did any planning, forethought, what if's, or anything of the sort. She lived her life as a series of desires and reactions to impulses. She had no grand plan and when called to the carpet on it she used the same inability to control her impulses to fabricate what she believed to be a clear cut story of what happened to her child.
I think most if not all of the information she gave family and LE was simply reactive (I have to make up something now) and not a planned story of the disappearance of Caylee. Sure, the minute thoughts were there, but she didn't have it laid out in any sort of manner. She was making up the story as things went along with family and LE. Not beforehand.
As far as whether she thought she could get away with it... I don't know, I don't know that she could even consider how much trouble she was actually in.
ETA: I have believed for a long time that Caylee's murder (if indeed committed by Casey) was committed in a fit of rage. As I've said before, children nearing the age of three can be quite difficult. Casey (imo) lacked the skills to properly deal with this and hurt Caylee in a way that couldn't be undone. Again, an uncontrollable impulse.