The crime occurred in March and she wasn’t arrested until September. Decoys might have bought her a couple days more time, but they didn’t even check the diamonds until the next day and surely would have discovered the ruse eventually. She was already out of the country before they discovered the switch, so it seems that she had time enough.
Also, just speculating, but I imagine there are probably limited numbers of places where you can get custom-made cubic zirconium jewels that closely resemble a 20-carat heart-shaped diamond so checking out those sources might have just given investigators another way to track her down.
