Galadriel's appearance is a little harder to justify, but not impossible. My guess is that she'll be involved in the White Council's efforts to deal with the Necromancer
at his fortress of Dol Goldur in southern Mirkwood. Given both Galadriel's longstanding involvement with the White Council, and her realm's relative proximity to Dol Goldur (I believe Lothlorien is only several days' hard ride south of the fortress), it would make a certain amount of sense for her to have a somewhat larger role than might be otherwise assumed...particularly if Gandalf & Co.'s efforts to take out the Necromancer is given more screen time than in the book.
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