I am sorry to disappoint some people here but force of 450kg is just NOT beyond human capability. I personally know people who can deliver this amount of force with a direct punch, not to mention a kick. One well trained person with enough skill and weight around 100-120kg can cause basically all of these fractures in the matter of minutes without "superhuman" effort. As for "bigger" size of the "hands" needed to inflict the injuries, the simple explanation is gloves.
Just ask a question: if I were to replicate those injuries, what would I do? Who would be technically capable of that?
My answer is that a well trained soldier or marine can inflict these injuries.
The link below has a table at end that shows what force can be developed in combat sports. Please note that these have rules, unlike military combat.
The figures presented in the link are in pounds not kg. The super heavy weight professional delivers a force which is only just over 450kg. I have already researched this and stated that only at the extreme end of human capability could Thibo’s depressed fracture be caused. Kicks and knees could also do this, but it would be a very unlikely coincidence that the shape was in exact proportion to ball of thumb. Also no human could deliver the forces required to cause Luda and Semyon chest injuries with a single blow as they were inflicted.
The attacker theory doesn’t explain why Kolevatov was left alive?
Yes the table is in pounds. What I am saying is that if there are (few?) people able to deliver that kind of force from a stance, it proves my point that this kind of force is NOT beyond human capability.
Please bear in mind that your calculation of 450kg is one of a static force (although I agree with your measurements in principle, to me they look mathematically correct). Secondly, they are approximate. Real number would vary from person to person. To calculate the dynamic force (more applicable if we are talking a blow), force has to be multiplied by acceleration. In other words you need less force if you deliver the blow while moving, running etc. The other thing to consider is that those measurements in combat sports were made with boxing gloves that are designed to lower the damage on the impact. The 'attacker' could have used tactical gloves which are designed to fo the exact opposite.
The "single blow" theory to explain the fractures is highly speculative itself. I explained earlier that rib fractures can be relatively easily inflicted during CPR by medial staff by repeatedly pressing the rib cage... and that happens every day. Just ask any paramedic. For this variant, much less power is needed. As for skull injuries, we have no idea what was the 'weapon' that hit the head,but we should not exclude an object in someone else's hands.
To be honest, I see not a single piece of evidence that would conceivably ruled out human involvement in the tragedy. Occam's razor tells us there is no need to include entities of questionable existence if there is a realistic explanation to the parts of the event. To put them parts together, however, is another matter. There are still questions to be asked.