I haven't read the whole thread, it is a tragic accident, one that a helmet may or may not have prevented.
My DS is a ski instructor, he has taught in Canada, India, New Zealand and many other places, he never ever takes anyone without a helmet, no matter how experienced they claim to be. My son also replaces his helmet ( at my expense!) if he has had a knock or fall.
It is compulsory for him as an instructor and he and his colleagues all wear helmets in their leisure skiing.
I have skied where Natasha Richardson did, it is gentle and easy, so this is just a tragic accident, that went sadly very wrong.
It looks as all the correct procedures were followed by the resort. I have hurt myself there, so have first hand experience of how they manage it.