I agree that it will be good for Dak to play directly under McCarthy's tutelage. This way there will be no doubt as to who takes the blame if they fail.
And it is possible that McCarthy can make Dak better; he's a solid coach.
But I think the result will be a permanent fracture between Dak and McCarthy- and a change will be imminent.
You talk about MM having a ball control offense. I just don't think Dak can run that type of offense because of his inconsistency and short game accuracy.