‘The relationship between Robbie and Cecelia would never have developed were it not for Briony’s false accusation in part 1’

I disagree. Robbie and Cecelia are already in the midst of a relationship when Briony makes her false accusations against Robbie. Possibly, their relatioship might have taken a different course. Maybe their relationship wouldn't have lasted through the separation of war, but it would have continued after Briony's accusations and may well have ended in a marriage embraced by their families. None-the-less, we will never know.