Okay, this is a little bit tongue-in-cheek, but ...
I'm firmly in the Cowl=Simon camp, but I thought of this earlier today...
Cowl is actually....
Mike the mechanic
Wait, think about this. His ability to return Harry a working vehicle borders on magic. Perhaps even resurrecting it from the dead.
His one, on screen, really big trick was moving a car off of himself that Harry had dumped on him. A Mechanic knowing about cars could do that.
And let's face it, even Harry has to make a living. Cowl would be no different.