I feel like the story will come back to these things. Life is not necessarily linear in spite of what stories tell us. Things don't necessarily happen in rapid succession, things happen simultaneously. We worked with the Stranger briefly, but got side tracked by advances from the Hive. That doesn't mean we will never hear from the Stranger again. And the Traveller is the implicit center of the story. Why are we under attack? Because of the Traveller. The fight is to protect the Traveller, the last hope for humanity. Details about the Traveller have been far and few between for now, but I'm guessing information will come to light with the continuation of the story arc.