I don't think you are right, but I understand why you think that way. Cecille is still fascinating, she is basically crazy. She is a box of contradictions and very few limitations. What happened is that the focus of the story moved away from her. It's not that she is not interesting, it's that the developer is not interested in her anymore. Her personality is crazy intense and has a lot of twists. She may have been able to do anything.
In fact, I thought she may do something about the little companion of Mars, that imp, can't get her name right now. Because at some very important point of the story, she noticed how important that little thing was to mars and the power she had over him, and Cecille said something along the lines of "I have to do something with her". And the scene ended, so I thought it was a glimpse into a possible confrontation between Cecille and the imp (homunculus?) to take over Mars.
But it never happened, either I missed something or it's just like I say: The developer changed his focus, started ignoring Cecille.
To be completely honest... am a bit disappointed with how the game came out. It seemed like it had unlimited potential, so many things could happen... but it seems like this is it, right? An entire cast of characters was created just to use their point of view during the end.
It's very weird, I have this feeling that something broke from the developer and crashed his game intentionally. Maybe he can't keep working on it or something.
Seriously. Unlimited potential.
If this is how it ends, I would go back, cut a lot of content and half baked arcs, and allow a more rounded and perfect art object to be released as complete. The game should have probably finished with the hero's mom fall. The perfect moment would have been once Sera and Cecille were corrupted but the mom brought a lot of great content.