I don't think the two cites are really contradictory, given Tolkien's old-school views: marrying a Man necessarily meant remaining with him/her in Middle-earth, and therefore staying behind when Elrond departed. I don't think it's necessary to pinpoint a moment when a "mortality switch" was flipped; Arwen does not appear to have died of anything except grief, and that essentially voluntarily.
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The entire plot of The Lord of the Rings could be said to turn on what Sauron didn’t know, and when he didn’t know it.
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