A couple of thoughts:
I don't believe Lureen was pregnant at all. I believe post-mountain, post-Jimbo-rejection Jack is counting his blessings that a rich girl is showing him attention. For him, Ennis is gone, flirting with cowboys or clowns is dangerous, and he is "starving", literally and figuratively. The backseat is his ticket out of misery, so he goes for it, perhaps engaging in self-deception for survival. But, Lureen is "on top".
I believe we're meant to see that as Bobby's conception. We all know there are lots of problems with the time-line, and I think this is one of them.
My very first impression of the happy sledding scene was how much Alma in her fleecy cap looks like Jack. Ennis even "wrestles" a bit with her, on a hill... sound familiar?
The "Alma flip" into bull-riding scene I thought showed a naughty Ang, but also reminded me of the line in the story "...Jack, who had been riding more than bulls..."
Not sure if Ennis's relations with Alma represent self-deception. Again, I think of the story: "I like doin it with women, yeah, but Jesus H., ain't nothin like this."