A couple of my own thoughts. I am often the only person, with a student, stopped to the side of a wide run, and still people come dangerously close when passing us. I have come to believe that it is the old adage of we go where we look. In other words, if I am skiing downhill and visually focused on the object I need to avoid, I will actually be drawn to it. This may sound familiar from bicycling. If I look at the post in the bike path I wish to avoid, I will likely hit it, while I need to actually look at the wide empty space where I hope to end up. The problem for us skiers and snowboarders is that the object is not stationary, so we have to keep looking at it to react to an unexpected move. Unlike cars on the interstates where we assume they will use a turn signal before pulling out in front of us when we pass.
Also, I suspect that big city people ski like they drive. Tight and cozy with those around them. Not their fault, but when space is shared, we learn to be comfortable with less personal space. Just a theory, but I see it on other aspects of daily living.
Finally, in terms of where to ski on the slopes, you are spot on about picking a lane and staying in it while skiing. As to which side or middle, I find that it depends really. I always coach my students to leave a wide enough slot on the sides for other to pass, as fast skiers often like to ski the sides and have nowhere to go when passing on the edge. The middle is often good for my slower students as it gives passers two options on each side. When I switch lanes or pass someone, I always look uphill as I do not want to get hit from behind. Who cares if I have the right of way, I do not want to get hurt in the first place.
It is also worth noting that in some resorts, the edges are the most dangerous, especially early season. Kids and adults that like to jump, often jump back onto the run from the sides, without looking first. Granted, they are 100 per cent at fault, but why even risk it.
I have also noticed that faster skiers tend to come in batches, even if they are not skiing together. It is worth it to pull over if at all in doubt about their skills, or lack of skills. This is where being good at movement analysis is a bonus and worth pursuing even for non instructors. I can quickly spot likely out of control people.
In 22 seasons I have only had two collisions of note. Both with my own students and I was hit from behind both times. They were the ones I was not watching out for as much. We can't avoid all risk, but constant reassessment of the people, and snow conditions, can minimize those risks.