1. I'm inclined to go with the idea that Marc was on a mission and already had the Steven Grant cover ID. The mission goes south and somehow we see Spector end up getting thrown off a great height.... And Then I think the Snap happens. He dusts before hitting the ground and then five years past and AEG brings everyone back. For whatever reason Spector came back as Steven, and ever since he's just drifting along. On some level Steven KNEW that Marc would be back.
2. What we see of Marc's fight with Hawke's men will be a flashback and Oscar's reactions we see aren't "the real world". Some how, some way, we are seeing Steven relive his time right before he falls over that cliff only we are seeing modern day Steven's POV on an event from his memory, just another side of his mind not something Steven remembers doing. Its a replay but Steven is experiencing it like it was real time without context.
3. Alternate theory... Marc is like a ghost, a possesion to Steven. He just blanks and suddenly he's in a situation, thus why he is tying himself to his bed at night. Perhaps the Hawke's men and truch scene is legit NOW. Question then is raised how he survives that fall?