Nice theory, though I think you made a couple of unsubstantiated leaps that create a really interesting back story... Chioma and Maya's story of love and separation, as it is written, wasn't meant to have a direct actionable effect on the story. Rather I think it was written to add some emotional depth (in this case love, faith, loss) to the lore. To make it relatable and not a narrative outline of what happened before. Because of this we have a human connection to these tales, it adds to their scale, and provides meaning. The exo stranger plays an entirely different role in Destiny. She is the "Herald" of your Hero's journey, she is the one that sends you on your way in the vanilla story line. I don't think she is another character in the lore, transformed, and then a part of your Journey. It make no sense (to me) for Chioma to do that. The Exo was sent specifically to you to tell you that you have a job to do. You are a chosen hero, messiah, savior, what ever you want to call it. The real question is Who sent the Exo