... BarbaraB, Karen G, and Frances G, I'd like to propose that Darcy knew from long experience that he had no hope of facing Lizzy in a full-blown confrontation, which is more than likely what would have ensued had he tried to speak face-to-face.

He knew what he had in his favor, but he also knew that she would have rejected it out of hand (as indeed she did). He knew that unless he presented his defense in a way that she couldn't breach by exploiting her greatly superior oral facility, he had no chance of being heard.