As for Elrond's foresight - he sees things that could happen. example: In return of the king, he sees Arwen having a child, but states 'that future is almost gone'; he can see the possible outcomes of the future, but none of them are neccesarily concrete. (I think...)

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<center> all we have to decide is what to do with the time that is given to us


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Elrond choses to focus on the fact that Aragorn will die, I don't think he cares so much about how long Arwen will live after that. Maybe he just thinks that since she's originally immortal, she will live as long as Elros did when choosing a mortal life - Elros became like, a thousand years or something, and Aragorn was just 88 when he married Arwen. I suppose Elrond just forgot that men's lives became shorter after the fall of Numenor... I don't know, though...

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by Lembas