Which is exactly why I don't think the tribes reuniting would work. Ember's earned her lock. Why would she give it up? Cutter told her to never undo it, as you said. Were they to reunite, I don't think they could lead together. If they had an argument, one would have to undermine the other, or damage their own leadership potential. I don't think Cutter could retire...he doesn't have it in him. And even if he did, those who followed him longer would be looking to him for guidance over Ember anyway.