I think some people are overestimating the difficulty of a software design change.
Having been part of a significant software redesign of a flight control system for a part 25 aircraft, which addressed a multitude of failure cases (including some we found in the course of the redesign and the associated design reviews) and which included some fundamental architectural changes, easily of greater scope than going from flip-flop alternating single input to dual inputs, and which took us from incident, through grounding, return to test flight, (re)certification and EIS inside a 12 month period, with frankly an order of magnitude less resources than Boeing can put on this task, I have to say that the timescales are more than achievable.
What appears (from the outside) to be delaying a return to flight status isn't the complexity of the task, frankly. It's FAA now going into complete CYA mode and every other decision during the MAX certification being dragged out and placed under a microscope. With the people looking through the microscope (who are not just the FAA, or even industry authorities, but every politician or journo sensing a news opportunity) sometimes having little conception of how the delegated/overseen certification process is supposed to work. (And has worked well for years)