There is another possible explanation, one which does not include any third party. But it's the sort of thing where readers will allow their emotions to revolt against the very idea and find it disrespectful to even consider.
Had there been a dispute at the tent, Semyon finding the siting madness and his greater outdoorsman experience recognising the weather was closing in, then this could explain some damage to the tent when a standoff and mutiny took place, and an old soldier led a better dressed party downslope to make a snow den in which to hunker down.
Those who went with him were the least fit of the group and entirely predictable in name, his new BFFL Tibo, and the 2 social misfits in this group, Alex and moody Lyuda.
The others remain at the tent, outraged at this, and Igor insisting they will be okay, that they have already dug a hole. Hypothermia slowly sets in and at some point they head downslope, perhaps determined by Zina when Yuri D begins to fade, and the party leave in a semi-dressed state due to the cognitive effects of the cold and make a fire in the forest, but this sees 2 of them die.
The remaining 3 are faced with a terrible decision, either lay down and die or take the sole remaining survivor resource, the den, which would be economically small and only fit 4 people. They cannot make their own, they are too exhausted and this would mean greater exposure outside the forest in rising winds.
So they'd take their torches and some fire logs and strike 3 weary and cold men in a hole in the ground across their heads to concuss them and drag them across to the ravine. Semyon's chest is flailed either as he hits the floor, or when Aleksander's unconscious body lands on him, this explaining the fractures happening during life. Semyon is found laid on his flailed right side.
Lyuda is spared being hit on the head, perhaps from an age where women were not struck, or they may have tried to convince her to remain with them, but she would protest, and to stop her screams, after removing her from the den as she scratched and kicked, one of them straddled her and pressed their thumbs into her throat, loosening her lingual bone until she eventually passed out, and she too was rolled into the ravine, possibly coming round again and trying to escape, sustaining a second set of fall fractures. She places her hat on Tibo after hearing slight groans in the darkness.
The survivors occupy the den, they are cold and in shock at what the human survival instinct has made them do and worrying how they are going to explain all this. Rustem has suffered a head injury during the fight, and Zina's hand is gashed when a knife was brandished. When the storm dies down they, or possibly just Rustem and Zina, head back to the tent but it's too late and they die of exposure. Igor follows, dragging himself out of the den.
The investigators suspect what has happened after finding the injured 4 bodies in a ravine close to a den for 4 people, and to spare the families, and to stop negative propaganda about Russians killing Russians, they close the case and prefer it to become a permanent mystery, one of thousands of State secrets.
When a group splits into two in an extreme environment they then become rivals for survival and will do anything to survive in what becomes a 'them or us' situation. This compelling force is not unknown.
I would of course prefer to think this never happened, but it, along with a poisoning, are the two theories I eventually arrived at in my own DPI journey, and perhaps internecine war in the extreme cold is the simplest Occam's Razor of all theories but is something of an unspeakable truth if that is what took place.
We'd all like to think that in those circumstances we'd behave differently, but I cannot say with 100% certainty what I would be like until I was there, and so I do not judge, and the hikers will always have my respect even if this took place, but when considering what might have happened all possibilities need to be considered.