Y'all are assuming Imu = Lilly is, Lilly herself actually deciding to be Imu.
Imu very well can be a brainwashed lili, dependent on what Imu actually is as an entity. If Imu is a bearer of great power, not combat but simply power, that allows the CD to rule the world properly or gives them some sort of an advantage in ruling the world, then Lili would simply be the vessel that was used to contain that power in.
The Five Elders have called the Nefertari Family traitors
The possibility of Lili herself just making the decision for her entire family with others in the family having no say, is very low.
She was the main leader, yes. But the family outside of her also chose to stay remain. Whereas they could've simply abandoned her and went to Mariejois, thus lili would be the traitor and not the entire family.