On the "deep amputation" quote: in context the narrator describes people running up a snowy hill followed by a groan. Isn't the meaning then that someone's leg or legs have sunk into the snow, figuratively amputating the limbs; the "arctic" in this context meaning "of or pertaining to the cold conditions which created the snowy hill?"

As to the purpose of the seasonal shift to Spring, if you go back to my piece you'll see I pointed out that christian imagery crops up quite a bit through the story, and that I narrowed the story's present day action down to the week before Easter Sunday. Therefore the perverse Last Supper episode is for the narrator then a precursor looking forward to Nina's death, a last supper before her sacrifice. The end of the story's important to keep in mind as well since the narrator declares Nina to be mortal, unlike Ferdinand and Segur--and unlike Jesus who rose from the dead. She's simply gone but for the memories the narrator can evoke of her which simply reenact the death.

By the way, on St. George theme, Nina makes the sign of the cross over the narrator's head each time she leaves him which is an important echo of the St. George story. I think if the mood seems pagan rather than christian despite the christian imagery it's because nabokov has inverted the meanings of the christian myths, Victor does not defeat the dragon Ferdinand and so Nina is sacrificed and dies, right?