I would offer Marvin the GM job but remove his head coaching duties. If he refuses, he's gone. I'd give first look for HC duties to Gruden or Zimmer for continuity purposes, but I wouldn't hesitate to bring in candidates from the outside.
Marvin has shown year in and year out an inability to make coaching adjustments and win a big game against a team with a smart coach. He is certainly better than some of the chaff we've had in the past, but he cannot take the team to the next level- he's maxed out at 11 wins and a first round playoff exit.
The Bengals will never get to the Super Bowl so long as Marvin is the coach. Unfortunately, they can't make a change at owner.