A live Denarian can do most things a human can do, and more, of course, depending on their special powers. Nicodemus could turn into a shadow and move around as a shadow. If he is still alive, I think he is a top candidate for the whisperer. Although, thinking about it again now, I wonder how any Denarian could get into the church with their powers intact. As you say, Uriel never confirmed it was a Denarian. Or maybe it was in Harry's head? Some remnant of Lasciel's shadow that wasn't totally converted by Harry?