Patriot1, you make some very valid points there, and I can see why a lot of people believe that Jack did quit Ennis, but, as I said above, I'm in the other camp and always will be. My belief stems in the most part from the line "I wish I knew how to quit you"....not "I'm quitting you" but "I wish I knew..." or to put it another way "I can't quit you because I don't know how". Yes, they've said a lot of hurtful things in that scene....and yes, I think maybe Jack thinks it would be better all round if he could go after that sweet life with someone else....but at the end of the day, it wouldn't be a sweet life with anyone else because most of what would make it so sweet would be being with Ennis.
As to why Jack told his folks about Randall (or about someone else, anyway) I think he did it out of anger, much the same as why he went to Mexico after Ennis's divorce. Who knows, maybe he was even goaded into it by his father - he's obviously mentioned Ennis often enough in the past and there has to be a pretty big chance that his father would have asked whether Ennis was ever going to come up to Lightning Flat. With the way he's feeling at that point, Jack likely would have snapped and said that he was going to bring someone else up instead....maybe meaning it (at least until he thinks about it in the cold light of day) or maybe not.
At the end of the day, though, I am 100% convinced that, had Jack lived, he would have met up with Ennis in November. He couldn't walk away after the confrontation - going to Ennis to hold him and comfort him when Ennis breaks down because he can't stand to see the man he loves in pain - and he couldn't have ignored the card Ennis sent. Maybe things between them would have been more difficult, but he'd have been there, I'm sure of it.