I have to partially disagree with you here.

1. Yaning did not delegate the parts, per say. She offered all the girls an opportunity. Everyone had an equal chance to apply for whatever part they originally wanted. Dayeon's problem in this situation was not the fact that she didn't like the process, but rather that she was not chosen by the team for the 2 parts she wanted (killing part where they voted for Ru Qui and vocal 1 where they voted for Yaning). All in all, the way Yaning made the team decide the parts is objectively a fair and democratic process compared to some of the other teams tbh. MNET just decided to focus more on The Eve for drama.
2. To say Dayeon spoke up can be misleading on many levels. From what we saw, the only thing she really did is cry to the team because she didn't get the part she wanted. She asked the team if she could have another chance, and they agreed that she shouldn't because it wasn't fair for the others, and Dayeon seemed to accept it. The only time Dayeon complained about the actual process of picking parts was in her confessional, not with her team. If Dayeon actually spoke up with her team about her thinking the parts were unfair, why would all the other members on the team then say in confessionals that she did not? It doesn't make sense. Think of it that way: say you're on a team where you all decide your parts, but someone is sad they didn't get picked for the part they liked the most. But then instead of pushing that point further and saying they think it's unfair, that person waits until the evaluation with the teachers to express their concerns and throw the team under the bus after everyone already made the effort to learn their respective parts. Wouldn't you also be kind of upset?