Thanks for that, KnC. Flying a Dak on that route in indifferent wx, including in and out of Narsarsuaq, would be (is) a challenge in itself. But doing it while towing a heavily-laden glider must have been a bloody nightmare, particularly when both aircraft were icing up. I wonder if the RAF Daks had deicing boots and prop deicing, as we did later on public transport (not that they were always effective). But the glider pilots would have had no such mod-cons, of course, as well as the absence of cockpit heating mentioned in that very good video.
In the event of an engine quitting, I guess the Dak would have had to release the glider and hope to be able to struggle on to its destination on one. That's when the PBY would have tried to earn its keep...