To your last question about the great journey: I believe it was the San'Shyum (the Prophets) who first discovered Forerunner technology on their home world as well as a few other planets and in doing so they discovered evidence of the "great journey" and misinterpreted this evidence as some kind of divine transcendence. They then used the advanced Forerunner technology to defeat and assimilate the other alien races that make up the covenant. The great journey was then taught to all the races of the covenant as they were assimilated and was used to help unify them. Not all the prophets believed that activating the Halos would trigger a divine exodus, some of them were starting to think that the Halos were weapons of destruction, but it was heresy to say this so they kept quiet. If you read the book Halo: Broken Circle, one of the main characters whom is a prophet starts to believe this. Halo: Broken Circle is definitely the book you want to read if you want more info on the origins of the covenant and the great journey. The book is basically about how the prophets fought, subdued, and assimilated the Elites, and the effects that would have later. And you won't get a better source of info on the San'Shyum as a people and the beginning of the covenant as a whole. Really good read if you are a fan of halo lore.