1) As I've probably stated before, I love looking at specifics in how language is used and how such usage reflects society. All through The Federalist Papers the phrase "the United States" uses the plural form of a verb—the United States have, for example. Now it doesn't. "The United States has" is proper. Guess the States have, indeed, become United. When do you think the change occurred?
The formal change occurred in 1861 when the confederate states seceded from the nation. President Lincoln held that to be an illegitimate act and the ensuing war settled the question in favor of unity. Clearly, the identity of the United States as a nation evolved between the 1787 constitution and the 1861 crisis, with regional differences in evolution speed.
