Re: he was unaware of the "defeat device"
He is the CEO, and ultimately the buck stops with him
Which is why he resigned. Too early in my opinion as, unless he was personally involved in a cover up, he'd be well-placed to lead an investigation. Playing musical chairs like they just have, doesn't really help.
However, legally I'm pretty certain that he is in the clear.
Seeing as other manufacturers have remarkably similar results I suspect it may only be a matter of time before similar discoveries are made elsewhere. Helluva a way to deflect criticism from General Motors recent failings.