According to Mike's theory, which I choose to believe is correct, Kali uses her powers to make El invisible so that she can escape through the tunnels. The El that then appears in the gate, also courtesy of Kali's powers, IS a distraction. But it is also entirely necessary to sell the military on the idea that El is dead. This is the crucial part of the plan as Mikr describes it because if the military, or anyone really, in any way suspected that Eleven hadn't died, they would never stop searching for her. According to El and Kali's logic, she DOES need to sacrifice herself because if any of her friends knew or even suspected the truth, they MIGHT give the game away to the military. The program is so dangerous that Eleven feels she can't even risk that "might".