So, after trying for quite some time I have given up for now.
I unscrewed the retaining ring in an effort to then be able to unscrew the optical block. After removing the retaining ring, I could not for the life of me get the lens block separated from the helicoid. I couldn't unscrew it by hand. So I used a rubber gasket. But the gasket only unscrewed the front element. The rest of the lens block still sits tightly in the focusing system. Once the front element is loose, there is nothing to hold on to left. Only the aperture ring sticks out, but I fear that I will break its tiny screws or even the whole mechanism if I hold onto that while trying to unscrew the block.
I did go into the lens from below. Once the retaining ring is off, the cam can be removed. Once that is out, you can take out a flat shim and three pins that keep the whole unit from turning. With these out, another large ring can be taken out that limits the turning motion of the helicoid. Once these parts are out, the lens and helicoid spin freely. This is a point where setting a mark would have been helpful. Of course I figured that out after the fact.
I gave the parts a cleaning since there was some gunk in the mechanism that could very well have limited the focusing motion. It was tricky to get it all aligned again when putting it back together. But I figured it out.
I have the lens back together now. The rangefinder seems to align better at infinity. The focusing action towards infinity is smooth again. The gunk-removal might have helped with that.
But when I took the bottom of the lens apart, I seem to have moved some dirt into the grooves of the helicoid or other places. Turning the focus ring now has uneven resistance toward minimum focusing distance. This is a bummer.
The aperture mark is still off by the same amount. I unscrewed the front element once it came loose during my efforts to unscrew the lens block. I cleaned the threads and put it back in. In order for the mark to align with the numbers on the ring, I'd have to screw it in more tightly than I am comfortable with/capable off.
With my twisting and turning, I have removed more of the white paint on the distance scale. It is hard to read by now, which annoys me greatly.
My findings summarized:
I learned that I have a lot to learn as a lens repairman. I learned a lot about my lens. Not all of these findings are good. The helicoid needs a professional cleaning. The binding of the lens block to the helicoid must be solved. I myself do not want to work on the helicoid while the lens block is still attatched.
For reference, I used this video as a help. It is very helpful in gereral (as is the whole channel), but it could not assist me with my particular issues.