Excellent point(s). Taking the arguement to extremes (or absurdity) to make a point probably isn't as necessary here as it is "some other places" on the internetz'.
Your one person cannot make a difference arguement however does fall flat if we take the next step. For example, you do not think 1 billion Chinese seeking the life-style we are so fortunate to be able to squander is not going to "make a difference"? What would be the effect if China had the same number of automobiles per capita as the United States? Conversely, what would be the effect if the avg. mpg of autos in the United States was raised 2 mpg. To affect change, things need to move in a direction, not a specific target (e.g. everyone does not need to drive a Prius or be a Vegan).
Oh, and saints/demons or heroes/villians are not born, they are made by the circumstances they find themselves in and how they choose to respond to those circumstances because everyone always has a little of both sides at all times, IMO.