Ali is as lead as you can get. The entirety of the film revolves around his relationship with Mortensen, and while it's true that Tony Lip grows more as a character, it's because he has a larger gulf to span than Don Shirley, who nevertheless comes out of his shell and becomes more of a social character. If Ali were a woman, it wouldn't be up for debate.
Simmonds is co-lead with Krasinski and Blunt. She has as much perspective as them, and her character probably goes through the biggest arc of them all.