Cowl is Harry Dresden.

As Simon Cowl? 1 - Very obvious. 2 - Harry does not know Simon. Harry believes that Simon is dead. He (Simon) may not appear in the Council. Why would he need to disguise his voice then? Or even hide from everyone? 3 - The Cowl height is never mentioned (nor Kumori) 4 - Does anyone remember the prostitute in DB, that it was strange looking at Harry. As if she had seen a version minutes before. 5 - Cowl can open paths in Nevernever. As Harry, now. 6 - The magic of Cowl is tarnished. As the magic of Harry.
Cowl is Harry from the future to something we'll find out later.

I kind of love this theory.  It would be really interesting if at some point Harry changed so much in his life that he kind of....split, much like Evil Bob split off from regular Bob.  And the Winter mantle stayed with the Cowl part. Ooo.

But it would be just like Jim to keep us speculating and then have Cowl actually be a new character