Or someone (perhaps one of her LE tied guys) knew of the stolen identity of ZG right around that time, and figured it'd be easy to make it look like the person who stole the identity then went on to kidnap Caylee.
I think once Caylee was 'taken care of' after the accident or whatever, then Casey worked with SOMEONE to concoct her story.
I believe the plan was to make it seem lke it happened later...post the 24th (thus the phone call to JG where she had him hear Caylee...which slightly makes me think the "someone" is NOT JG and he was another person she used) and have it look like something happened to Casey and Caylee both (thus explaining the car and purse abandoned). I think they expected the car to be found sooner, people to panic, and Casey to turn up/be found having "escaped" but Caylee having been taken...pinning it on the person who had robbed ZG.
When that scenario didn't work, they had to adapt it a bit and not have Casey appear having escaped from this person...unless that's where Cindy says we'll see Casey as a hero, if Casey's story would go on to say that she fought this person, etc. and fought for her daughter, but they got away with Caylee and Casey escaped.
From the little her friends have said, they thought Caylee was with "the nanny"...none of them said the name. I think the name came later on. Pin it on the person who stole the identity, and now you've got an anonymous bad guy using someone else's identity. That'd stall LE first looking at all the ZG's before someone might then connect the dots and then have to find the thief.
And that could explain her distrust of local LE. What local LE person would believe one of their own gave the identity of the person who had the identity stolen over a girl who's claiming someone did?