Question: The road leading up to Minnie's has been freshly plowed when the stage arrives. It is obvious because there are sharp borders at the edge of the road and no tracks. Did the bushwhackers plow the road to eliminate evidence of their nefarious deeds?
Answer: Yep, gotta be a mistake. If they even had snow plows back then, the plow would have had to been between the bad guys and the stage and would have arrived, or passed by, at a very inopportune time. I think the director just missed this, or didn't think anyone would notice.
Answer: Think you mean the plow would have arrived before the stage. But yes, a mistake for sure.
Answer: Good catch! This should be a revealing mistake entry. The plowing was part of the set preparation. I saw several scenes in the film where the roads had obviously been plowed. It's not hard to see if you look for the small ridges left by the plow on either side of the road.