I suppose not, but it has been said that the film fudges "Stonewall's" views on slavery to make them more acceptable to the contemporary audience. If that is true, it could be cause for concern.
That is not why the South fought, in large measure.
It depends on what you mean by "the South." If you're talking about the average soldier you're probably right. If you're referring to those who led the secessionist movement, you're probably not.
I saw this movie on the 16th. I don't remember hearing Stonewall ever mention slavery.