She wouldn't have told such a whopping great lie and have everyone out searching and doing the whole acting "frantic" to the police thing if it wasn't for a sinister reason, IMO.
If it was some sort of accident and she was scared of social services, the worst that could have happened would be her kids would be taken away, and she'd possibly receive a small jail sentence. She was taking a massive risk making up the whole disappearance thing, forensics would have most likely counted against her. If she'd been found out the jail sentence would be far longer and she would have lost her kids anyway. I think she would have weighed this up if she was innocent and told them that she was sorry but there'd been a terrible accident, not run this mad wild goose chase.
But IMO, she's a narcissist, and actually probably quite a stupid one, just like Casey Anthony.