Why this movie felt dishonest to me - spoilers

Just watched it last night, and unlike many of the posters here, I did NOT see the twist coming. But unlike The Sixth Sense, I don't think the plot twist was used fairly, for one main reason: The housewife needed to be an Oscar-worthy actress to pull off the ruse. There was never a false note or a point in which you could tell, even in hindsight, that she was faking. Perhaps if they had a bit at the beginning about how she used to do community theater before having a kid, or something like that, it would make the leap more plausible.

Also, the whole elaborate plot could've blown up if certain things didn't go exactly as planned - things that required her husband to act a certain way. One thing is that they BOTH had to sign to withdraw the money, but somehow she sends him back to the car and SHE collects the money. If he had stayed with her, she would've been forced to withdraw all the money. Or if he had demanded to speak to Sophie at any point. Plus, what if he fell off the ledge - they would've been murderers at that point.

The movie was ok - it definitely held my interest, but the hingsight 20-20 review just doesn't work.