Ok, I'll just tell this one part straightforwardly enough cause it's hard philosophical sci-fi:
In my interpretation / expansion of the Marathon story, time travel is accomplished by stepping outside the universe entirely - not into another universe, but into a wholly different level of reality. Like if a monster in a video game somehow managed to leave the video game and, not come out into our world, but start running around in other programs and other parts of the computer. It's not leaving the computer, it's just leaving the confined of the part of memory it was programmed to run in, and becoming capable of reprogramming that and other parts of memory. Of course a monster in a video game can't just be free code running in the computer and still be a monster in a video game, so to leave the video game and remain what it is, it must somehow create another "bubble reality" for itself, program a mini-game for it to exist in when it's not running around inside of an existing game.
That's what I interpret the "dreams" in Marathon to be; when you step Outside, you bring your own mini-universe with you. Since it's drawn from your own mind, it's like a dream, but you are making it real, which is why you can still die there. Of course, since you are now no longer just a character in the game but sort of a self-programming, self-playing game, you can just respawn again, so that's not really the death of the real you, just that body in that one universe. AIs, having no bodies in the first place, exist as just pure thought when they go Outside, seeing all the different physical universes as equally real (which drives most of them mad). They don't have to conjure physical universes for themselves, but they can still affect any physical universe ("real" or imagined), and so they can speak to you in your "dreams", or if you are a n00b at this like Marcus is, being pulled Outside rather than just going there voluntarily, they can create a dream-universe for you and plop you down in it. In the case of the Aye Mak Sicur variants in Infinity and the Inti Station variants in Eternal, the dream-universe is a vision of a possible future world; but it is not actually that world.
So basically, every time you are dreaming, you are time-travelling, or otherwise not a part of the normal space-time continuum. You have left the physical universe, but being an essentially physical being, you conjure your own little physical universe around you, or someone conjures one for you, and for that we have no better English word than "dream", though it's not really a dream like you have when you're sleeping.
