1: are there examples of the public preventing or adressing shootings in the US
Tons. But using us as an example probably won't get you what you want. The U.S. is a huge country with more than 60 times the population of Norway, and an incredibly diverse one at that. Because of that diversity, and the greater disparity between top and bottom economically speaking, we have always had a higher crime rate and probably always will. Would more or less guns change that? No one can say for sure, but the prevailing wisdom from people who should actually be taken seriously is that no, it has no real impact.
When Britain virtually banned guns, their gun crime dropped, but the overall violent crime did not. It actually increased, though it is purely speculative to say that the ban was causal.
Criminals will be criminals, and they will use whatever tools they can find to do their bidding. In the US, firearms are more available, and thus more commonly used. In the UK, firearms are far less prevalent, and so thugs use knives, clubs, fists and other improvised weapons. Statistically speaking, you are less likely to be murdered but more likely to be a victim of violent crime in the UK. This has not changed much since the days when Britain didn't have such strict controls; Like Norway, the UK has always had a lower homicide rate than the US.
The Homicide rate in the US waxes and wanes a bit, but it seems to do so completely independent of gun laws and ownership/CCW at that time. The economy seems to be a much more important factor
Basically, what I'm trying to say is that both sides are wrong. More guns does not equal more crime, nor does it equal less.
However, restricting ownership of firearms by law abiding citizens means denying them the ability to defend themselves using one of the most effective tools we have. I'd much rather live here in the US and face a higher risk of being murdered yet have the ability to defend myself than live in the UK or elsewhere where I'm more likely to survive an assault, but also more likely to be assaulted and have no means other than my own physical strength by which to avoid it.