This wasn't what happened. When Ginti told her to choose either Harada or Light to drop into the void, she chose Light. When they got into the elevator, Mayu asked, "Where are we going?" If she chose to join Harada into the void, she obviously wouldn't ask that question.
Ginti then answered, "Where Harada's soul is." This can indeed be interpreted to mean she knew where they were truly heading because she had a rough idea that his soul had been sent to the void before, but her next line wouldn't make sense if that was the case. "Will he really wake up?" she asked. Ginti had already explained to her what kind of place the void is. The void is a graveyard of souls, where only the consciousness remains. Taking his body there to wake him up makes little sense with that in mind.
That conversation implies that Mayu did indeed drop Light to the void. By doing so, she believed she has saved Harada. Ginti then tricked her and told her to drag Harada's body to the elevator and pick up his soul to wake him up.
That was why the elevator mask was fake. That was why Mayu was so shocked when they started becoming dolls. That was why Ginti was so satisfied.
He managed to "pull out" her darkness by forcing her to make a ridiculous choice. There was no way for her to let Harada go into the void, even if they were to be together. She loved him too much and was too selfless for that. By making it a choice between Harada and some other fellow, Ginti compelled Mayu to make an "evil and selfish" choice.
That's what I meant when I said that the horrors of the judgment system was brutally portrayed. It has no fairness or meaning. Ginti is right. Arbiters judge just because they do. They aren't equipped, deserving, or even truly willing.
I agree with you about the conclusion. Mayu got a semi-happy end because her soul was joined with Harada's, and that, at least, sort of rewarded her dedication.
EDIT: Additional evidence that she dropped Light (kudos to the creators for this) into the void:
At the beginning of the elevator scene, Mayu was looking at her right hand with guilt. It was probably the same hand she used to press the red button. There's no other reason for the animators to draw that scene like that.