Ethan, what a gift this is, to bring us back to those incredibly moving scenes - thank-you, for letting us go there again.
Here's my own look-over: in the SS, although Jack loved the memory of the DE, it had also become a grievance, because he saw it as Ennis being afraid to accept him, face-to-face, as Jack, in his arms. The key words about the memory itself are "later" and "solidified". So he had a memory of a real event, but kept it tied to a grievance. In the film, there's a different grievance. Ennis has accepted him as Jack, his lover, but will not commit to a full life together, on a daily basis.
The film uses that grievance to show the startling transformation of Jack's face from the sleepy contentment of the DE, to the jarring anger of their last meet-up. It seems to be used to explain why Jack was going to make his move, and try for someone else to move up to Lightning Flat. He had come to the end of his resources, in holding out hope.
By the way, if I can extend the timing of the DE in the movie, to the following flow of events, some have said a year passed before his death. But AP has the last meet-up, where he remembers the DE, in May, which is still Spring, and Jack's father refers to Jack as changing from Ennis to someone else, in Spring, so the intensity of that anger-gaze suggests immediate resolution.
The screenplay notes seem to have errors suggesting a longer period of time, before the postcard was returned. But it all seems to fit better, that Jack made his move in May/June, resentful of the loss of the August meet-up, and dies before November (which Ennis had off, and which would allow for Ennis to go to the Twist farm, after the death).
The timing of remembering the DE in the film, then, was, IMO, used to fade into the harsh gaze, and then a cascade of events. If that is true, the memory of the DE, may well have triggered the attempt by Jack to set himself and Ennis free of each other, which led, maybe, to his death.
It was raw beauty, yes, but it may have been used in the film as the beginning of the end. Jack's hopes had run out. (But I think, had he lived, he would have dropped his grievance and gone back, to try, yet again).