Why did Aharon stand between the dead and the living?
Rashbam To prevent the Malach ha'Maves from crossing and continuing to kill.
Seforno: To cure those who had already been affected by the plague and were ill. 1
Targum Yonasan: To Daven that the plague should cease.
How exactly, did Aharon stop the plague?
Rashi: After scattering the Ketores between the dead and the living, he seized the Mal'ach ha'Maves and convinced him that, despite the fact that Hashem had sent him to destroy, Moshe, who only carried out the instructions of Hashem, had sent him to stop the plague. 1
Rashi: As implied by Pasuk 15 "va'Yashav Aharon el Moshe ... ".
Seeing as the plague already stopped when Aharon stood in between the dead and the living, what does the Torah mean when it writes "ve'ha'Mageifah Ne'etzarah"?
Seforno: Refer to 17:13:1:2. It means that those who were already ill were completely cured.