Nickf, my only point was that you appeared to blame the person in front for changing direction, and suggested that the person overtaking at speed was being hit by a poor skier (in your first example of poor skiers causing problems)
And that is 100% not the way it is.
It’s the uphill skiers responsibility to avoid the downhill skier, there isn’t any grey area I’m afraid.
And if you ski alongside someone, at the same pace, and you collide then it could be either person at fault, or 50-50. Which is why that’s best avoided.
(if we are counting, I probably have somewhere over 400+ weeks skiing, and a fair amount of those I was teaching. And it the first and most fundamental rule that would be taught)