I think it's likely that they Agent and the Abiter are working together, because if you look at the Agent's visor near the end of the trailer, you can see Arbiter's reflection. I'd suspect it would only be for a few levels, I think every core Halo game should be about Master Chief's story, and focus on him.
This means all campaign missions will be unlocked. Everything is unlocked, right from the start- Clarified in an interview to mean the campaign missions.