The Justine/Claire schism did strike me as somewhat arbitrary, but the fact that the structure of the movie tries to be clearly divided is kind of a reflection of the film's pervading duality. Life/death, happiness/sadness, indifference/caring, Melancholia/Earth, and Justine/Claire. I think Trier is toying with opposites throughout the film, and splitting the film into two chunks is perhaps a way of further exploring that.
I am surprised, though, you think the Claire section was more unnecessary, Bailey. Arguably that's where most of the plot happens, and the whole Justine section doesn't concern itself much with Melancholia the planet at all.
I do think the theory that Justine is a sort of prophet figure is accurate. Perhaps Trier is implying that depression and the indifference it entails allows people to see the futility of life or something, to have greater insight into it. And that insight is what allows her to provide a last few moments of contentment for Claire's kid by providing him an avenue of escape into fantasy. The only chance you have when faced with death is retreating into your own mind, which Justine is naturally suited for due to her the introspective nature of depression. I don't know if that's true, necessarily, but it seems like what Trier may be getting at.
Also, I do not really think Justine ended the movie "happy" in any way. She looked content in the last few moments in the "cave," until she sees Claire suffering from intense grief and terror right beside her. Then she actually starts to crack and almost looks like she's about to cry before Melancholia hits. In a way, Claire's sadness transmits itself into her, which is what Justine has been doing to everyone else throughout the movie. So maybe, in a way, the movie is actually about a depressive becoming self-aware and overcoming their depression by understanding how it affects others. Emotions, then, are not such an individual thing, but are what connects and binds people -- which is ultimately sort of an optimistic view that suggests we are not actually alone. Maybe the last few moments of the movie are about Justine realizing that.