Well what we see here is the stage 3 top, already assembled with stage 2. On the left pic you have a Briz space tug on the foreground, which is the 4th stage used for commercial satellites launches. We have to check but I don't think there was a 4th stage for the Mir modules.
The good news is that I found some good stuff on Anatoly Zak's website. Of course right now I'm on the test articles and there is no known detailed footage of their assembly (it probably was one of the most top-secret things of its era). Still, I now know roughly how stage 2 and 3 are/were assembled. After the 4 test-flights they extended the 2nd stage interstage, so that they had room to fit a third stage with a decent nozzle and this is pretty much how the Proton-K is born. The Proton-M features improved engines, up-to-date electronics and structural optimization, but is other than that very similar.
My test article is now 41.5 meters tall, and this is very close from what it should be (the most accurate reference I found is "more or less 41 meters"), so I think we're good. I don't think it has separation motors, the quality of the pictures is very poor but we would notice them ; also they were unnecessary to deploy the Proton satellites, springs were enough for that.
I'm processing the first stage for Orbiter exporting, lot of mesh cleaning to do and of course this is where I notice differences with reference pictures. But I'm understanding better how it works, and that helps a lot. Also unless we find conclusive evidence of the contrary, the first stage should be common to all the Proton versions (I'm pretty sure there were some small differences, but we don't have enough material to figure them out, and Thorton didn't either).
The 4th stage shouldn't be very hard to do because 1) I already have a Blok-D model from the Energia 5 project 2) ILS Proton user manual provides very nice blueprints of the Briz upper stage.