The crew had gone insane by the end of the filmIts implied throughout the film that staring at the sun for extended periods is playing havoc with some of the crew's mental stability. As such, to gloss over the inconsistencies with part of the films ending/Genre Shift, it would certainly fit with the theme of the melancholic character studies and subtext that Capa (or any of the other crew) had eventually been driven insane by either the futility of their mission or by the sun. This would certainly explain Captain Crispy from Icarus 1 still being alive after X amount of years with 100% skin damage. Anyone who died as a result was either via their own mistakes, or in fighting (as we saw earlier in the movie, some were already at each other's throats), and any references to a fith crew member was in their own heads. Whether they still completed their mission or failed is down to the viewer - The Sun being reignited really happened or everything was simply Capa's Dying Dream.
The air in the big room the explosive material was radioactive or full of harmful pollutantsIt's always bothered me that they're short on air but then go into the bomb room and can breathe fine, this is my justification. They knew they could breathe the air there, but due to the toxins in the air it was being held as a last-chance air reserve for when the mission was an absolute "we're not going back".