Dr. Vladimir Borzenkov's explanation is very plausible. Although it is not necessary to depress the trigger to rewind the film on many 35mm cameras, it may have been required on that particular camera. It's also possible that the tech did it out of habit or to test the camera. In any case, it's absolutely impossible to tell what is going on in that frame. The circle in the center of the frame is, as Borzenkov stated, lens flare from whatever is going on to the left. It could very well be a photo of a flare or bright light taken on the slope in '59 or, it could simply be a photo of a light in a dark photo lab. There are 1000 different explanations that can be offered as to what it is a picture of. Too bad film isn't time stamped the way that digital is today.
Some knowledgable posters are certain that Eagle is a genuine photo (time2fly's profession is image analysis). So if one of Semyon's photos is clearly genuine, what was he trying to capture on the other frames? Something important to warrant carrying the camera around his neck?
One accidental exposure in the lab is possible but two? (#34 and one attributed to Rustem).
I would bet that all of Sasha's photos were ruined from the elements. Most of the frames where 'objects' are visible, the objects are no bigger than a couple mm and are found along the edges of the film near the sprockets. "Mushroom with a face," for example, is about the size of pinhead. Why would he frame something so important along the edge as opposed to the center. I would be that all of those 'objects' are just artifacts from developing water-damaged film. In any case, it is impossible to tell what any of those photos are. We're not even sure if the film had photos on it. The lab tech could have been 'developing' an empty (or partial empty) roll of film with absolutely nothing on any of the frames. Sasha probably left the tent with the camera because he probably had it around his neck when the event occurred.
Why should film be destroyed by immersion in pure meltwater? It's processed in harsh liquid chemicals at warmer temperatures, and even after it's dry the negative strips, or the entire film, can be washed and dried again, perhaps to remove dried-in dust or rinse fixer residue. Cameras have been dropped in the sea or lakes and when recovered the films develop.
Objects do not have to be snap-shotty, filling the frame, to be genuine. Taking photo's of the night sky, and from a mountain top, is unlikely to have any nearby subject filling the frame; this does not automatically invalidate all landscape photography.
Most of Semyon's photo's offer no valuable information, however there is no way that a film which is supposedly ruined can produce an image like the following, which is cropped, sharpened and colourised. There's even the 'dry ice' line of the light shining down on the fuller frame.
And it seems to me it is the lens of an electric light, looking not unlike the Stanley headlamp off my old Suzuki trail bike for that matter, or any number of similar headlights. It can only be one of two things, one far more likely than the other, and that is either a helicopter, or the headlamp of a snowmobile or similar if the photo was in fact taken near the forest, either of which means a military presence, in Soviet Russia circa 1959.