This OP was obviously meant to be cautionary, but I can actually see some, or all, of these things happening. I think the organization is really high on Jason Garrett and that they'd prefer to keep him if they can rationalize it. I think Kiffin probably retires. I think Marinelli's the obvious candidate to him if the departure is amicable. I'm a bit torn on Callahan. Not a huge fan of his, personally, but he and Garrett do seem to have a good working relationship.
Beyond the fact that we're stuck playing too much zone, the problem with the team isn't really coaching, it's personnel. I think they'd rather spend another year trying to seriously address that than bring in a brand new offensive system for their franchise QB and their third new defense in three seasons.