I'm reading all your posts with interest. You've really made me question whether I think the SNIT fits with the characters in the film (even though I don't think it does fit with the book at all). I think I said, that yes, I can see it fitting, but I think that's partly because it's there, so I have to MAKE it fit.
The reason it can't fit with the book is that they didn't embrace face to face on the mountain, and their sex was quick and rough with no words spoken (almost the opposite of the SNIT!). And that allows for Ennis to avoid facing something that he can't deal with at the time - that he has the hots for Jack (rather than having sexual relief with him because of the lack of women). So the big difference the SNIT makes is taking that aspect away - I get that. Ennis knows he has the hots for Jack, and that that they aren't good friends. And other parts of the story have been changed to support that (particularly the motel conversation). I have to leave that aspect aside when considering whether the SNIT fits with the film characters, and think whether this more knowing, more aware Ennis would have this more tentative, romantic sex instead of the straightforward, matter-of-fact sex that I think I expect of him.
I don't FEEL that it fits, but it does kind of make sense. The rougher sex the night before is all about Ennis exploding with a pent-up desire which he's been trying to avoid it. The FNIT and the morning after get that out of his system - the deed is done and he somehow comes to terms with that. The SNIT is like another beginning, as if they're discovering each other for the first time as lovers, even though they have had sex ... and of course, they are sober and aware.
It's likely that Ennis's limited sexual experiences (which might only have been kissing, but I'd guess not full sex) were with Alma, when they were "courting". My guess is that they would be more like the SNIT than the FNIT (partly because he wouldn't be overcome by passion and desire, but would be more tentative and exploring). It's even possible that Alma took more of a lead than Ennis, if she was attracted to him. Perhaps it makes sense that this second time, when Ennis knows he wants Jack from the start, that he'd automatically repeat what he's done with Alma, with Jack? After all, he later repeats what he's done with Jack, with Alma. That's in the film too.
I was also thinking that film Ennis has that quality of being like a horse with a "low startle point". Right from the beginning, he seems awkward, anxious, avoiding eye contact - all due to his background, but indicating somebody who might need to be more gently "managed" and seduced. That's at odds with the FNIT, but I think that in the film the FNIT is different - it's not the beginning of "how it would go", but more like a one-off. In the book, Ennis seems more relaxed, and they happily spend a night together without incident (at the beginning of the summer) - the FNIT seems to come out of the blue (although we know that it has been built up to). In the short story, there is no first night together and Ennis is a bit avoidant right from the beginning, as if he's aware of the risk. I also thought that Jack's soothing behaviour in the SNIT fits with the conversation beforehand, where Jack is reassuring Ennis. Jack grabs Ennis's left hand again, but this time ends up holding his face instead.