I, too, am sorry to have cut-and-pasted your full response but it does afford me a chance to discuss your observations.It was a case of luck they survived. Judgement wasn't lacking with those two because the wind, winter conditions and terrain caused the potentially fatal accident.
Could a GPS have saved them?
Learning how to use a compass well along with a map is something your are likely taking a lifetime to get right. I know I am. Do you not think that could be the same for a GPS unit?
... there are strengths and weaknesses to traditional and modern. You simply have to learn how to use either system and both systems preferably. Not many do that.
I remember when I did both my ML and Winter ML courses and assessments, the navigation instructors at The Lodge were very careful to point out the hazards of a cross-wind and the effect that it can have on your walking and navigation, particularly when on rough terrain. When visibility is compromised, as it obviously was in the case of the two about which you speak, it is even more important to pay very close attention, particularly if you know that there are drops nearby.
I'm not sure that a GPS could have saved them. Although my experience of the GPS technology is very limited, I am aware that, in poor weather, the devices struggle to latch on to satellite signals, thus compromising the accuracy of their locating abilities. Would such a device steered them away from the cornice? Possibly...
Like any skill that we use in the outdoors, time spent honing it and perfecting it is an integral part of the pleasure these activities afford us. We will probably never be 'perfect' but the joy of improving is most definitely worthwhile. I would love to get hold of a good GPS and learn how to use it properly and effectively; as you say, learning about both the trad. and modern systems gives us many more options in the outdoors and narrows the chances of our making mistakes. When time and cash permit...
Yes, there are also occasions when 'luck' has been a lifesaver but I would hate to rely on it. For all the shortcomings of any navigational system - think, here, about a compass's weird behaviour when on gabbro on Skye... - they do have the effect of rationalising your thinking at a time when conditions and stress may blur your judgement.