A spoilerific theory on the ending
I don't think either the simulation or true disaster theories really work.
I think that everything that happened after the oxygen depletion event going forward was a shared hallucination. I think the inoculations were in some way responsible for a shared unconscious delusion. It needed time and proximity for the bond to develop. Elements of the beginning were all effectively suggestions on their dream. Things like seeing the press people again (the freshest faces in their minds) and comments in the interview like 'testing the chemical properties of dust' - all things that contributed to the delusion.
So in my theory they never left the bunker. Their shared desire to leave the capsule started that narrative and the low oxygen created an environment in their minds based upon a low oxygen event on the surface. They all remain in the delusion for quite some time, but when two of them break out of the shared delusion they are erased from the narrative. What actually happens is that the events in the town are occurring in the capsule and eventually the two astronauts still in the dream end up murdering the two that escaped the dream right before the simulation would end.
All they had to do to confirm this would be to show a surveillance camera view inside the capsule that showed one of the dead townies to actually be one of the disappeared astronauts. If they did that the IMDB rating would probably be 3 points higher!