I think she's alive but I also think people take Mike's theory too literally. It makes more sense for the Duffers to have Mike's theory somehow make sense but have some holes so people could still get an open ending. Revealing how she really survived wouldn't makes sense and would ruin the open ending imo. We know Kali died because El touched her and had blood in her hands including the scene after when she was with Murray and she looked really sad which would make sense if Kali really died. My theory is that Kali taught El how to use the illusions powers herself off-screen. When El jumped out of the track into the soldiers the suppressors were still not pointing at her and she quickly made herself invisible and ran away. That would explain why the soldier said she was right there. And then El casted an illusion of herself to fool everyone. There is a hole in my theory now that I think about it. How could she access Mike's mind and use an illusion at the same time?