That really does get to the heart of the disaster. Lets explore the question. There is nothing in the case file to suggest anyone slowed the progress of the hike after Yuden departed. Igor's ski boots being cached is something that seems wrong though. So, I will assume for sake of arguement that when they left the tent on 1079, all were equally fit to travel. They must have had only two priorities. They were heat and shelter. From the available evidence the heat produced by the fire at the cedar was insufficient to heat them all up as a group. One forum contributor suggests that excavating a snow cave happened at the same time as fire building, hinting at an early division of labor. It also suggests a relay from excavation to warming and back to excavation. I suggest Zolo was in charge of the cave.
If three people abandoned this process to return to the tent, I can only imagine it because the weather had moderated. That said, I think they would retrace their steps for safety and efficiency. This did not happen. I think it odd because the prints were preserved. This hints that the weather was not that much improved, but it also suggests desperation. Again, a conundrum. What was so compelling that they couldn't attend to when they all first left the tent?
So, was Igor first to die? I do not think his state of dress is an indication. IRZ may have acted independently of the others who in their sub group, shared tunnelling tasks and clothing from the Yuris. If so. The Yuris may not have proceeded IRZ in death. For me the only indicators for IRZ are the time it takes for the body to shut down, the direction of travel and injuries. I do not see any cardinal indicator that Igor, nor anyone else was first to succumb. My intuition says that they all fought the urge to sleep until each in their own time did so.