1. We learn in the film that those with nose bleeds are being controlled; therefore, Cameron Diaz learning that the tuition discount is going to be taken away (not that she'll be fired) and James Marsden not getting his job (he obviously did not fail the psych exam) are conditions being put in place by the very people giving the Box, not just a convenient story telling coincidence.
2. Cameron Diaz isn't kidnapped (there is no shot following Langella making her fall asleep to indicate that) and in fact the son alludes to how she got home. (If there is any confusion in the editing of that "climax" it is how how how James Marsden got out of the car and into the NASA hanger, but that's a little later.)

I feel as if this dismissive review has its points of validity – this movie is messy, there are very obvious editing flaws – but, ultimately, in terms of Richard Kelly story-telling, however ridiculous the premise is, there is very little that is left unexplained (and that which is is ultimately inconsequential). What's more, it's worth seeing just for Frank Langella's stunning performance.

one of the worst things i've seen this year.

The premise is completely ridiculous. People die if I go to the supermarket. People that I don't know die everytime I leave the house. So, what is there to convince someone to not push the button?
The film actually addresses that. During the point in the film when the couple are deliberating over whether to push the button, Marsden's character even says aloud, "people die all the time". There's more to The Box and its mystery than that.
Not much, though. As this review suggests, the moral is completely obvious and the film works through a lot of useless padding to get there.
Actually, that may be my biggest complaint about the film: almost everything between the pushing of the button and the conclusion could have been cut and it would make little difference to the message or emotions we get out of the film. I know that 'it's the journey and not the destination' and the journey is, as the review says, mildly compelling, but in the end it's also pretty superfluous.