I've always been at Red at fault - while Grey might have widened his line he was doing wide turns which got wider rather than say switching from a fall line schuss to a lateral carve. In fact I've always been mystified that Red never saw him at all and makes not the slightest reaction. Of course Grey's failure to keep watch is what contributes to getting him creamed. Without a video in a lawsuit it's hard to see it being clear cut , Red "he swiped straight into me from the side", Grey "He was blasting down the slope like a maniac".
Re the POV video, there just isn't enough context to see where the snowboarder comes from. I'd guess he overtakes you and cuts in front - still his fault as he hasn't completed the whole of the move by the time you (hypothetically) hit him. But if he's overtaken you when you're going to the left of the safety banner say and you hit him then it seems to become more your fault.
The way I think of changing "course" might need a significant change of vector (of course this is the wrong term as any turn is a significant change of immediate vector). An example might be schussing the cat track in heavy traffic then suddenly swerving one side to the other.
I'm totally on board with what you're trying to debate here. Clowns will be clowns and will hit innocent people ,or to a lesser extent but relevant to those of us being responsible, set off or merge straight into someone's path. Rightness under code or SSA doesn't stop you being in ER, and certainly doesn't ensure that there will be enough evidence that you recover damages etc. Neither does it necessarily protect you from being on the losing end of a lawsuit once emotional things like "loving mother of 3 children", "knocked down" "beginners can't be expected to be in control" are thrown in.