You know, I absolutely understand what you're saying, except for one major thing: I don't think that Ennis had any choice in the matter. I mean, I sincerely doubt he sat down, ever, and said is there a way we could be better off? That trauma was so severe that it actually limited the growth of his personality, to the extent that he didn't like "thinking," or at least what he meant by "thinking" - "You're a real 'thinker,' Jack Twist," is said as if it were an accusation. The real trauma of this horror deprived him, in many ways, from developing past certain ideas he held as a 9 yr old.
If you doubt this, the next time you watch, listen carefully to his conversation with Cassie in the bus terminal. He has made up his mind by this point, in fact made it up the night before the fight, that he was going to dump her. Just look at the expression on his face that night as he looks at Jack - he has openly got it for Jack from that moment on, and Jack doesn't get it! (Remember, these guys started out each agreeing that "I ain't queer." That's the 'contract' that let them continue to have sex on the Mountain, and they believed each other's lie, and it's that lie that stays in their way for 20 years.)
"Sometimes I miss you so much I can hardly stand it," is a revelation to Ennis and the death knell for any further doings with Cassie, and this is why he is such a mess, poking around those poor little apples from his pie: he has already come to grips with his and Jack's reality and is anxious about seeing Jack again {the postcard has been sent} before Cassie comes into the terminal. This is the reason he hasn't been contacting her the way she wanted - "You must a got all them notes...,"and the reason he looks so anguished and confused - he really isn't sure how Jack will take this revelation. But his answers to her pain are not the answers of an understanding adult, or even of a weaselly adult, but just imagine a kid saying them - they fit a kid's way of responding to a scolding. Even when he tries to take some of the blame for her pain, "I probably wasn't much fun anyway, was I?" it's a kid's answer, and is also why he is so totally floored when she says, "Ennis, girls don't fall in love with fun." This is the first moment in his life when he finally understands that love isn't just the fun that he has with Jack, it's his pain,and it's Jack's pain, and the knowledge that Jack still tries to comfort him in his pain, too. You can practically see him understanding this, looking into the future where he can tell Jack he loves him. This is why Cassie is actually so vitally important in the film - she is the first (and only) person in his life who gives him any clue as to what love is. This is also why I doubled up with that gut punch of "deceased" in the very next scene - Ennis finally knows, but doesn't get the chance to say. :\'(