Some updates... I managed to repair the "screwed up conduit". Actually most of the run wasn't original anyway, only the short stick from the junction box up to the ceiling was Bergmann conduit, the run inside the ceiling was PVC, so I managed to replace that piece with PVC. In the living room none of the wiring collides with the plumbing, so I did the complete roughin. 1976 to now the room only had outlets on two walkls, now there are two on each wall (originally it had one in total, the second one was in the kitchen, other than that the place had none... no wonder one circuit was enough).
I also had to disconnect the kitchen as we removed the lowered ceiling and the light feed attached to it (exposed conduit). That means the old wiring is by now reduced to the yard light and the outbuildings.
Turned out the wires in the cellar weren't actually live - the mystery splice wasn't a splice any more, the wire ends had been taped, folded back into the conduit and plastered over... apparently the original splice had just been in a hole in the wall with a cover slapped over it. And one of the cords was actually 3w+ground, so no illegal happenings here.
Considering how many circuits I'm going to install I wonder whether I should throw all my principles overboard and run at least the feeds up in the attic and then down into each room (that's not the way it was done in the good old days, and since I consider wiring of the good old days very easy to trace and neat to work with I usually try to follow those old guidelines).
