This is one of the first questions any investigation would look into. Do we have any materials from '59 or modern, that answer this?
Who saw them alive last and when? When exactly did Yuri Yudin turn back? There is mention in the diaries from the time at Settlement 41 that he decided to go home. Did he then still ski to 2nd Northern afterwards and turn back from there? They spent a night there, did Uncle Slava who helped transport their backpacks on a horse-drawn sled also sleep there and then took Yudin back the next day? Isn't it strange to decide to go home due to pain but then still ski for a day, even if it was without a backpack?
Even after another person saw them alive, they took photos of each other. At what approximate time were the last photos of each member of the group taken?