- Red conductor (old GND) used as corresponding wire for a 2-point light switching.
Actually no violation. VDE was much less strict on the use of red wire than it is now with yellow-green. Old 3-way switching systems often used the red and grey wires of NYIF as travelers (Korrespondierende). 4 wire 3ph wiring used red as a phase color.
Those setscrew connectors are used along with choc blocks and often regarded as the worst crap ever made. They're called "single box clamp" (Einzeldosenklemme) and it's pretty challenging to wire hem in such a manner you don't have hot wires poking out on one end. If you try to too many wires into them some will invariably fall out. I only use them in emergencies when a box is really too tight and I can't replace it. Most electricians here still use choc blocks.
We had that type of junction boxes in the basement and upon rewiring I just removed the terminal block (bakelite in that case) and thereby converted it to a modern box. I wasn't really keen on using 50-year-old terminal blocks with open screws. Choc blocks offer some level of protection, even with the box cover off. This style of j-box was pretty common for basement wiring from I guess the 1950ies to the late 60ies, then most of the brown/black stuff was replaced by light grey. Originally they were used with heavy metal sheathed cloth wire (2 or three rubbber and cloth covered conductors, a thick layer of cloth, a layer of sheet metal and a final layer of asphalted cloth, about the thickness of a water pipe, when I first saw it I thought it was 16mm conduit). Usually brittle as hell, can't work with that stuff unless you wrap all wire outside the sheath with electric tape. I believe it was called Ku(h?)lo cable.
The cut blue wire is absolutely ugly, all he'd have needed to do would have been opening the screw and pulling the wire out.
BTW, what color is the wire next to the cut blue one? Yellow?
And black and grey wires under one screw don't look too good either.