Hey, Bud! Good to see you -- it's been a while, for some reason, and I'd begun to wonder why our paths hadn't crossed lately. Now, as usual, you have drawn me in with an intriguing thread.
In fact, I just referred earlier tonight to the Jack/Alma rivalry on a thread called (I just looked it up; I've been posting so promiscuously lately I couldn't remember off the top of my head) "Interesting post from the imdb board," on CT. This was in connection to something later -- the idea that Jack may have felt jealous of Cassie, even though Ennis clearly wasn't very concerned about Jack's fling with "the rancher's wife." I think this difference is also illustrated by Jack/Alma (tension) vs. Ennis/Lureen (little or no tension). Ennis doesn't care about Jack's male/female relationships, but Jack does.
I agree that Jack is staking his claim with Ennis back on Brokeback, with the laundry being the metaphor. Ennis, for his part, seems oblivious (just as, later, he seems OK with Jack's fling with the rancher's wife). In the reunion meeting between Jack and Alma, Ennis seems too happy and excited about Jack's presence to notice any undercurrents. In the "now you leave Alma out of this" he's just being fair and chivalrous and -- YES! -- Jack's look is very intense and I've always wondered what that was about! I think your idea that it's the rivalry emerging is exactly right.
I also am enlightened by your take on the Thanksgiving scene. I always interpreted it purely as Ennis being angry at being outed but, you're right, the "Jack Nasty" remark must have stung as well.