Watching this episode again, and pondering the oddity of an entire planetary population transplanted into their own past, it occurred to me that...maybe they weren't. Is it possible that perhaps the people were actually deposited into a (for lack of a better term) "holodeck" of sorts? More specifically, like the Moriarity-in-a-cube situation. It seems more merciful to allow the Sarpeidon citizenry to believe they had escaped an inevitable doom. Of course they would all die when the sun exploded, but until that point they wouldn't be counting down the moments to destruction in fear. Think of the old people in Soylent Green. It's a bit like that. Mr. Atoz' absolute dedication to waiting until the last moment to go back himself suggests an unthinkably high devotion to duty. It's possible his people were just that believing in mercy. The very existence of the Atoz replicas does suggest a high level of 3-D imaging technology. Though, how this Zor Khan (sp.) character fits into this is a mystery. As for Spock, it may have been possible for him to devolve mentally to the state of his ancestors in a type of mind-over-matter situation, kind of a reverse situation to the one in Spectre of the Gun. There he knew the bullets weren't real, therefore wouldn't affect him. Here, he believed he was five thousand years on the past, hence his psychological reaction. This all would explain how the population traveling into the past didn't alter the planets' history, as well as how not being "prepared" by the atavachron would allow our heroes to return: they had not been "digitized", so to speak, into the system.