I'm suggesting this with little evidence supporting it, but by process of elimination, I don't think much else is left.
The radiation report needs an explanation. I have good reason to believe it could not have been caused by a small tactical nuke, which I'll post about later. Contamination from work or school wouldn't affect all four hikers in the ravine. And the investigative team knew to check for radiation in the first place, which seems extremely odd that they'd find it.
Let's assume the military was conducting a test of some type of bomb spiked with Sr-90. I could easily see this being a thermobaric bomb, and the purpose of the Sr-90 is to act as a tracer. Then, the military can come in after the fact with beta sensitive Geiger counters and chart where the shock wave hit by the presence of the radiation. (This isn't a terribly environmentally friendly thing to do, but it's also in the middle of nowhere.) Alternately, the military could have been experimenting with "dirty bombs" with the express purpose of delivering radioactive contamination.
Sr-90 would be a good candidate for a tracer. It is produced in abundance in nuclear reactors, so it should be easily obtainable in quantity for the military. As a beta emitter, it isn't going to give any staff sent in to monitor it a gamma dose, which would be far more serious. Sr-90 has historically been considered an excellent candidate for a dirty bomb. There have been reports from WWII that Oppenheimer discussed poisoning Japan with it
The military has no idea the hikers are present. The noise of aircraft and/or bombs detonating roused some people from the tent. Then they realize they are very exposed in their current position when something detonating nearby indicates they need to evacuate immediately. Hence cutting out of the tent and evacuating in various states of undress. Evacuating down slope is natural, and the trees and/or ravine may have offered what seemed like more cover.
These activities are responsible for the UFO sightings. It's possible Krivonishchenko's frame 34 could have been of an aerial detonation. Military tests may have repeated on several nights in different areas, again explaining the UFO reports.
A thermobaric bomb may have caused the treetop scorching. It is also possible that a shock wave from one caused the internal injuries to the hikers found in the ravine while not producing comparable external injuries. The Sr-90 in the bomb caused the contamination on their clothes. If something detonated in the vicinity of the tent, it's possible the tent could set off Geiger counters, too.
After the military testing was over, the four hikers in the ravine were dead or close to it. The other five may have survived and either tried to keep warm with a fire near the cedar or attempted to make their way back to the tent. All of them eventually succumbed to hypothermia.
Meanwhile, the military remained oblivious to the fate of the hikers. I chalk this up to a big bureaucracy where the left hand doesn't know what the right hand is doing. A screw-up of this proportion, e.g. accidentally killing 9 college students in a secret weapons test, would be beyond awful. If the military had any idea what had happened at this point, the 9 bodies and the tent would have conveniently "disappeared." I think there is no way the military would have allowed the bodies to be found if they had any say in this matter, with that much potential for forensic evidence giving something away. Likewise, I can't see the military letting the hikers be found with cameras and undeveloped film. If the hikers and the tent simply disappeared, would this forum exist and would we be talking about them today? Probably not.
I think, at a minimum, the search and rescue team was underway and close to the hikers before the military made the connection between them going missing and the bomb tests. In that case, the military couldn't slip in to "disappear" the bodies without making their presence obvious.
Once the military realized what had happened, and that it was too late to make the bodies disappear, they arranged to have Ivanov installed to lead the public investigation, presumably because they knew that for some reason they could either manipulate him or that he would be sympathetic to their cause.
At some point, the military also realized that contamination was likely and they let Ivanov know about this via some kind of back channel. They may not have wanted Ivanov's team getting contaminated or otherwise endangering themselves. So a little "*hint hint*... You might want to take a Geiger counter to work and test stuff" may have happened.
This explains one of the biggest mysteries in my mind. It seems surreal that someone would both possess a Geiger counter and randomly decide to check the hikers for radiation, after which it turns out they were contaminated with a weird beta emitter?
I think it may be possible that the tip to Ivanov to test with Geiger counters may have been a later development. We only have a radiation report for the four in the ravine, which leads me to wonder whether the first five were buried before the "tip" arrived. I also recognize that the timing of this is uncertain.
The radiological report on the four hikers in the ravine raised more questions than answers. But, acting on pressure from the military, Ivanov did not have further tests conducted, and instead closed the case. Also, access to the area was supposedly closed for three years. While Sr-90 has a 29 year half life, over time it may migrate out of the environment. So limiting access to the area for a few years while things cool down made sense.