Your point about the twin threats coming from the Left and from Islam reminded me of an email I received from Fr. Schall some months ago when I shared a draft of the Syllabus with him. He made the same point, as both the Left and Islam are voluntarist systems where will is exalted over reason. He called the parallel between them the main issue of our time. Many of the points in the Syllabus were paraphrases of an earlier Schall essay on voluntarism.
Benedict is not denigrating Islam or its prophet but setting forth a theological problem, one that arises within Christianity itself, namely the problem of the tension between the intellectualism of Augustine and Aquinas and the voluntarism of Duns Scotus. "Is the conviction that acting unreasonably contradicts God's nature merely a Greek idea, or is it always and intrinsically true?" Roughly, does the transcendence of God -- which both Christianity and Islam affirm though in different ways -- imply that God is beyond our categories, including that of rationality?
Perhaps a better way to put the question would be in terms of divine sovereignty. Is God absolutely sovereign and thus unlimited in knowledge and power? Or are there logical and non-logical limits on his knowledge and power? For example, is a law of logic such as Non-Contradiction within God's power? In his 2012 Creation and the Sovereignty of God, Hugh McCann argues that God is not only sovereign over the natural order, but also over the moral order, the conceptual/abstract order, and the divine nature itself. That seems to give the palm to voluntarism, does it not?
I consider McCann's view to be highly problematic as I argue in my long discussion article, "Hugh McCann on the Implications of Divine Sovereignty," American Catholic Philosophical Quarterly, vol. 88, no. 1 (Winter 2014), pp. 149-161.