‘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.

I agree, to a certain extent. If we look at their relationship development, all this attraction and new-found passion happened in the space of a day and there is no telling whether the passion that they had for each other would have stood the test of time. The one big obstacle in front of them would have been their class difference which is a major theme in atonement, with Robie being the daughter of the Tallise's charlady there is a very high chance that Cee's family would have disapproved of the union and they probably would have been separated by this class separation. So in my opinion the false accusation of Robbie led Cecilia to choose between her love and her family, a choice she may have not been able to make had Robbie not gone to Prison. Their correspondence over letters over the years strengthened their relationship and nurtured a bond that I think they would have never been able to cultivate otherwise. Cecelila's letters kept Robbie grounded during his worst times at Dunkirk, by giving him something to look forward to. In conclusion, I believe that Briony's false accusation of Robbie not only strengthened their relationship, it caused it to develop into what it was towards the end of the novel.