Apollo 8 went to the moon and the crew didn't turn into xenomorphs. I don't think they were quite unsure what to expect out there but there were lots of other factors in the scheduling.
What I mean is I don't believe either of 'they had no idea what space would do to them' or 'they were just following some military convention' had much to do with the way they scheduled crews.
This is sort of what I was getting at with the phrase “abundance of caution”. At this time we were still quarantining people who returned from the moon.
One thing we do know (and knew at the time) is that the earth->space and space->earth transitions require a period of adjustment. The added stress of repeating this adjustment has, so far as I know, still not been tested, let alone understood.
Yes of course. But as a factor it would be far lower in importance than lots of other much more basic ones. Like, say, the fact that the missions were quite different, you have to prepare crews for them, you have to prepare backups, you have a limited set of highly trained, highly ambitious overachiever volunteers to pick from, all of whom expect to fly (in history-making missions), etc, etc, etc.
