Marianne, as I was preparing this question I found another long-ago question about gun control, maybe it was from Midnight Cowboy?
Anyway I cannot find the question now, but in it, Other Tink made the observation that Switzerland had a relatively low homicide rate, at the same time gun control very lax. The point being (I think) that Switzerland has better mental health? ...and she mentioned it would be interesting to hear from you about that, however I don't think you posted an answer to that particular question...do you have an observation about gun laws and gun violence in Switzerland now, by chance?
I did not go through all your links, your thorough workup, however just from living 73 years experience I have some theories - for example, both parents working and latchkey children. My own generation was, I think, the last one raised with a parent always at home. I was unable to open the Telegraph link, but that title mentioned lack of role models for children.
I recall when I learned to use guns, safety safety safety was rehearsed and emphasized endlessly.
I also feel the increasing use of drugs has been a factor increasing violence.