For people asking about what happens in the original novel, that answer is… not a whole lot. At the time, the series was almost entirely told from Rae’s perspective, so all she did was spout exposition while Claire did the food drives and negotiations. Most of the actual revolution happened in the background. I’m sure part of it was Rae keeping Claire away from any unnecessary action scenes while making sure she only got involved in ways that would put her in a positive light.
All the other scenes the manga has shown are new additions that could have happened in the background, with the exception of Arla turning on Salas, only to be assassinated by her own people. That was something that definitely did not happen in the novels. As far as I can tell, she allowed Salas, the man who destroyed her family, to hijack the revolutionary government as a means to an end, even if it meant selling out to the Nur Empire. Considering her only real goal was to abolish the aristocracy, and she doesn‘t do anything afterwards, this change actually paints her in a better light. It lets her maintain some integrity, even if she has to die for it for the rest of the plot to happen. Her “role” in the final conflict isn’t necessary and can be filled by anyone else.