Simon makes the most sense.  I used to be part of Team; 'Cowl and Kumori are Harry and Molly from the future' but recently re-read Dead Beat and had the idea knocked out of me by the most obvious reason:  Cowl engages Dresden outside the bookshop while Kumori sneaks around to Harry's side of the pavement to whip the book from him, and Harry sees her.  Molly uses veils as second nature - it could never have been her.