But the Protocols of the Elders of Zion is a proven forgery. The White Horse Prophecy is a documented statement made by the founder of Mormonism.
That's hardly the point, now, is it? Due to the continuing revelation nature of the LDS faith, many of Joseph Smith's statements are no longer canonical (a quite sensible way to run a Church, preventing you from getting stuck with the bizarre dogma of your founder due to a new revelation coming down the pipe every so often). The LDS even has officially backed off somewhat on the Nephites/Lamanites thing, saying that they were only the ancestors of some of the Native Americans, not all as the BoM insinuates.
As regards the White Horse Prophecy, it's not and never has been official Church dogma, and even if it were, the idea that the United States Constitution would be saved when hanging by a thread is not necessarily a prediction of theocracy (any more than you could pull various bizarre Papal bulls throughout the millennia and say that, say, "The Syllabus of Errors" makes Newt Gingrich unsuited for the Presidency).