Q1: Technically, she gave up her immortality 38 yrs ago (when she excepted Aragorn's proposal), and is only now going through with her plan. I think why Elrond says "until the changing of the world" is Fran, Phillipa, and Peter trying to fit in the lovely line in Appen A:
Quote:
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...And there is her green grave, until the world is changed, and all the days of her life are utterly forgotten by men that come after...
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Q2: Because Jackson wanted to add more tension. That's the only reason. And the only explaination! Curse him!
Q3: They exaggerated that it was the "last ship" of course. And I think you're explaination works well. But throw in something about Vilya and the other of the Three and you'll confuse them enough that they'll stop asking questions whose answers lie somewhere in Jackson's fuzzy head...
P.S. love the movies at all, but I'm a purist.