Yes, I have to come down on the side of the "one shot" being the summer. Ennis couldn't let go, everything had to be self-contained, under control. He feared losing control "if this thing grabs us...". Up on the mountain they were free, free to be. Ennis ' engagement is a committment that he had before Jack, one that he must follow through on coming down from Brokeback. It's also a natural cut off to the unleashing of his emotions towards Jack, stuff it all back into a box and be a "real" man. There's a day coming when he will go back to his everyday, normal life, but for now he is free to experience this new, frightening thing that has overtaken him and simply enjoy Jack; to revel in loving another human being as he has never known before. That's why he's so angry when Aguirre brings them down early. He's not ready for this "one shot" to end, he is not prepared to lose Jack now.
Rancher? Did I read rancher? The rancher in Texas I feel was very much a current thing with Jack up till his death. The story has him at it with other men during those four years straight after that summer, but the movie paints a picture of a man whose heart is broken after Ennis gets in touch about his divorce (Why? Did he hope for just a moment that this just might be his opportunity to really be with Jack for keeps, only to lose his courage by the time Jack drives all the way to Wyoming?) and so Jack drives off to Mexico and finds comfort where he can?
So, too, the bit where his dad says that Jack had big plans to bring this Texan up to the Twist homeplace to start a new life après Lureen. Dad knows what this is all about, he’s fuming. But was this Jack’s response to that truly horrible scene that really ends it where Ennis is on his knees in such pain that he begs Jack to “leave me be”? :'(Was Jack really making a final break and going to leave Ennis? I wonder. In my mind there was either a grieving rancher down in Childress missing his buddy, or maybe he was throttled with a tire iron too. Six Days Later and Still Crying, Titus.