Was DeMornay setting Cruise up from the beginning?

I think she was involved. The first time he leaves her alone, she steals the egg. The second time he leaves her alone there is a montage of her ransacking the house, and looking at all the valuables. I don't think anything in movie shows any type of growth or change for the character. There is also the fact that Guido knew exactly where they were and what they were doing when he robbed Joels house. If it were as impulsive as Lana made it seem, no one else should've known about it. Also Joel called Lana to tell her about the robbery, and Guido picked up her phone. Joel does confront her about it, so even he is suspicious, and her no answer isn't very convincing.

As for why, I think she just saw an opportunity. Earlier in the film Joel described her shameless pursuit of material gratification. She was much better at capitalism than even him. While Joel was falling in love her her, she just saw a naive kid with some money, and connections to other people with some money, and she looked for a way to exploit it. She is the one that came up with the idea of getting their friends together. She created the situation where Joel needed money by locking the keys in the car. I believe she was also in on a scheme to get Joels share of the money as well.

She remained cold and emotionally detached throughout the film. There is no reason to believe that she felt for Joel the way he felt about her.