After listening to the Felser interview, bringing back Marv make total sense, only not as a GM or coach.
Remember that before TD was hired, the Bills never had a "President" -- only a VP of Football Operations.
Right now, Ralph wants someone who he knows and trusts, and who will be loyal to him. So he brings in Marv as President to "stabilize the organization" and "clean house" (Felser's words), to get the ship pointed in the right direction. Then, Marv hires (or promotes from within) a GM, or VP of Football Ops, or whatever. Then a new coach is hired.
The more I think about this, the more I like it.