Well, at last I tried a careful approach and it seems it worked. I had the benefit of founding on the web a picture of the Plaubel Makina II rangefinder withouth it's cover, so I knew what to expect inside. I've not adjusted any rangefinder before, but had seen several videos and knew the general working of the system.
So first I unscrewed the two small screws on the top of the rangefinder housing, and it came out neatly and without anything (like a spring) flying away, which was my main concern.
I've attached a couple of pictures I took of the mechanism. Looking from behind the camera, the semi-transparent mirror to the right is the one which moves with the focusing arm. But it does this like folding it so was not clear at all how to do the adjust here. On the other hand, the left side mirror has several screws pointing to it's back. Gently moving them clockwise or counterclockwise (just a quarter or even less of a turn) allowed me to correct the vertical and horizontal disalignment.
Now at least it seems to be correctly aligned at infinity, and "more or less" at nearer distances. Still work to do, but the general movements of the rangefinder are these.
Any ideas?
Some pictures of it here under: