When he reentered Roland's world it allowed for him to have been at the way station and to have been killed by Roland. It is the moment of discovery. Before that it is Shroedinger's cat both alive and dead at the same time and then upon his reentry into roland's world it allows for him to have been where he had and hadn't before. Yes, he was at the way station, and went under the mountains; but he also didn't. The paradox is negated or resolved by him being pulled by Roland through the door.