This doesn't sound as terrible as it looks on the surface.
Kelly can stay and still keep around a lot of the concepts and plays that Deshaun likes and is familiar with, much of the terminology as well. Meanwhile Daboll can implement a better playcalling philosophy, the 'whys and whens' of the playcalling. A better opening script perhaps too. This would also free up Daboll to be a full on HC monitoring all phases and keyed in on time management, situations, etc.
And though I'm not particularly thrilled with another rookie DC, if Mayo is the guy to light a fire under some asses then by all means. And as long as I'm staying on the bright side I know he had a reputation as a tireless student of the game type with a mean work ethic.