I've been thinking about something similar to this--it doesn't have to necessarily be a cover-up. It could have been a "shut 'er down."
What if "an unknown compelling force" was the specific answer, just not named? What I mean by that is, those in charge of the investigation are given the direction of, "Look, we know what happened to these kids. They were forced out/extreme force was used" and we're just going to leave it at that." The old, "need to know basis" and no one involved in the investigation really needed to know. Someone at the top knows what happened, whether accident or murder or what have you, but all anyone really needed to know was that it was violent. It happened. And now it's over.
I speculate this only based on the political nature of the Soviet Union at the time. Eliminate all our questions, all the shoddy evidence and theories. The USA in the 50s had their own witch hunts going on, trying to sniff out communism and ruining peoples lives/careers because of it. The Soviets were so quiet and tightlipped about everything. Yeah, they can't hide the accident at Chernobyl. Gotta fess up to that one. I just don't think a political machine like the Soviet Union would really need to go so far to create a cover up, especially if they knew what happened. If they knew and determined no one else really needed to know, then they just ended it. They acknowledged it was overwhelming, no need to specify, and that was that. Kind of like, you're not exactly lying, but you're not exactly telling the truth.
And, in the end, no matter what specifically that force was--be accident or murder or Yeti or rouge Mansi--politically/culturally speaking, I wouldn't think they'd want that kind of information getting out to the public. I don't know much about Soviet Russia, but I do know that information here in the States was pretty regulated in the 50s. Assuming it was the same in the Soviet Union, it's reasonable to assume they want everyone to know, hey, we're in control. We've got this. There's nothing out there that you need to worry about, nothing that's going to chase you down a mountain and leave you to die. They gave the explanation and quelled the proverbial fire before it started.
Of course, this could just be late night pondering. It's interesting food for thought. A socio-political perspective of Russia at the time/what the general populace was allowed to know verses not allowed to know, would be interesting. Would it help solve the mystery. No. But it would perhaps give context to the world our nine were living in.