I think there's enough evidence to prove that Zanny could not have placed a dead Caylee in Casey's car trunk.
Casey told LE that she never saw Zanny again after leaving Caylee with her on whatever date was supposed to be the last date Casey saw Caylee. We know that Casey had the car after her last supposed encounter with Zanny. And we know that Casey left her car at the Amscot, where it was subsequently towed and then finally re-possessed by Cindy and George.
There's enough evidence there, and enough of a chain of command to rule out the possibility that Zanny came back to place a dead Caylee in the car trunk. Of course, the jury will ultimately decide whether to accept this interpretation of the evidence, or whether to accept whatever theory Team Casey might raise at trial to challenge the idea that no one could have come back to put dead Caylee in the car trunk.
Some other poster suggested that the defense will claim insanity. This appears to be the defense's best option. Whether or not they would succeed is another story. This is an extremely difficult condition to prove, and I'm not suggesting that the defense will prevail on this ground.. But a successful insanity defense would tend to sweep all of Casey's problems away with one neat explanation.
Otherwise, Casey has tangled herself into so many knots that she really has painted herself into a corner. Her position is like those Rubik's cubes where it's impossible to put one piece of the puzzle into place without undoing two or three other pieces of the puzzle.