one clue that they were still all on the ship and never left it

toward the end of the movie, the house looks very greenish and it moves in waves like a hologram! that is how I knew that what they were "seeing" was either pre-programmed to give them these illusions to deal with or they were sharing a hallucination.

It would not be past the testers to rig up some situation far different from what the astronauts are told they are experiencing.
400 days is longer than a year. does it seem like they were in there a year? no.

I think that part is part of the illusion too. wherever they were, Walter knew they would try to leave at some point. and whatever is out there is manufactured. what they heard during their first 100 days was them creating this fake world and town. and yes, some of the actors were possibly people they knew already.

Remember. they should not have been able to open the hatch. they were supposed to be in space ad certain amt of time. they would not have opened the hatch is space. so the fact that they could open it is weird and walter planned for that.

Again, I am saying a lot of what they saw when they walked outside was fake due to the way the house looked. if the world was rained down upon with moon dust and the sun was gone, those people would have frozen by that time. Fake.

The guy they killed kept saying lets go back to town. that might have been a rescuer Walter planned for in case the astronauts could not get back to reality by 400 days. but they killed him.