So, I've opened the switch and inspected - it is in perfect condition, although had few dead cockroaches from 90s (compared to the switch, they aren't ok though).
Then I've turned on the line and poked everything with multimeter - everything is ok. Bulbs also don't glow after switching off, so looks like problem is gone, at it was related to moisture. Still don't understand what really happened, but I guess it will wait until proper rewiring combined with home repair that will surely happen sometimes.
>He says back in the day it was the standard that they tied the Ground and the Null together in a box (most likely where the circuit breakers/fuses are) but standards changed and now one of those are counted as Line too (he said which one I forget, it doesn't matter to our point, I think it was the Null however), >If the tubing gets wet, the moisture in the form of vapor can go through the whole system, also causes corrosion. Somewhere the damp closes the circuit for you. You should check each connection box to see if the Ground and Null are connected together.
Yes, looks like it is the case, or at least it is similar to this. In old USSR-made homes there were no ground, only two wires go to each socket, and null functions as ground or such. Old wall sockets have no ground at all, except in special electric stove sockets. I've replaced few old sockets recently, they all had two wires, so I've installed new ones also with disconnected ground. As far as I know, line had upgrade some years ago and there is ground at dashboard now, but only there.
>Real solution would be to rewire the whole thing...
Yes, it is inevitable thing. Old wiring aren't good at all, it is also aluminum, not copper, wires lost flexibility, corroded etc. Thing that stops me is that everything must be broken, from parts of walls to ceiling, and it is best to combine that process with complex renovation (from wallpapers and floors to furniture). This is costly and time consuming, and flat must be emptied from everything first, but I have no other place to live now.