I see - you're "solving" the problem with the label in the author macro here: you never actually use it. 
It's preferable to let the "substitute" do its job. I don't think this will matter greatly for the style as it is, but right now you don't have provisions for a book that has an author and an editor ( think e.g. Augustine. The City of God Against the Pagans. Edited by R. W. Dyson. Cambridge Texts in the History of Political Thought. Cambridge ; New York: Cambridge University Press, 1998. ) Once you allow for such book editors, you'd get the editor twice for edited books. Using substitute: http://citationstyles.org/downloads/specification.html#substitute gets rid of that.