RouxB, youx and Ix ARE onex! I agree completely with everything you said. And Amanda -- thanks for joining the discussion! -- I agree with everything YOU said, too. (Including the part about the romance-killing line from the story; I'm so glad the movie doesn't play it that way.)
I SO wish the filmmakers had showed what happened between the argument hug and the truck driving away, but like Amanda I think this is another example of their exquisite torture. The "damn you Ennis" I don't mind so much, because I see it as just another way of Jack saying, "I wish I could quit you" -- he's frustrated, Ennis is driving hiim crazy, damn him, but he can't let Ennis go.
What bothers me more is Jack's "I did, ONCE" -- that past tense makes ME feel awful, so I'm sure it couldn't have been pleasant for Ennis to hear. I think, as both of you suggest, that the argument probably leaves Ennis feeling worried and scared. As for Cassie, he just doesn't care enough about her to keep up the charade when he's already distraught and depressed. He has been pretty confident of Jack's love all along, now he realizes it's at risk, is terrified of losing it, but is still paralyzed by his homophobic fears.
Not until the shirt scene does he realize that their love was so great it should have been allowed to outweigh all other concerns. (And Amanda, I also like how mutual their feelings are for each other! Though it seems to defy cinematic convention, which usually calls for uncertainty, in this case their being sure of their own and each other's love somehow makes their relationship more romantic.)
As for the flashback, the circumstances surrounding it are so unbearably sad that, sweet as it is, it is far from my favorite of their love scenes. The contrast between the hopeful promise of their relationship early on and the grim impasse they have reached is just too depressing.