From the sounds of it, it's a combination of the positive reception Daud got, as well as having a choice of two different characters for the main game and wanting them to reflect differently on the events as they occur. So Emily doesn't just have different powers to Corvo, she also has different opinions and commentary on things. Increasing replay value is always good.
Edit: apparently it was always planned for Corvo to be voiced in the original, but they left it until far too late in development to do it. Don't know about that...sounds like bad planning.
"Low chaos" is the canon ending for the game and the DLCs, right? They've not said much about Daud at all, but I figure if Delilah is back, Daud has to figure into this somewhere. Probably as DLC again, is my guess, though it would've been nice to have him as a main character option.