Great post brokeplex!
I definitely agree that the artificiality in both the interior design and the clothing of Lureen and Jack might be a sign of their false marriage.
As the years go on, Jack seems to "dress-up" as a cowboy, but he no longer engages in activities that would make him a cowboy and his outfits get more flamboyant in a certain way (flashy watch, really colorful shirts, etc.). And a degree of artificiality seems to creep into his wardrobe. Note the much bigger black hat at the end. He no longer seems to wear real working-cowboy gear. For Jack, being a cowboy is already a fantasy that's escaped him by the time of the reunion in 1967. He proposes a "cow and calf operation" as if this is some ideal form of employment and not part of his reality and from the story we know that by '67 he's already quit the rodeo. Whereas for Ennis, we know he already works for a "cow and calf" operation both in what he says to Alma in the grocery store, in the shot where we see him feeding cattle, in what he tells Cassie, etc. For Ennis working with cows (I'm being very literal here in my definition of cow-boy) is a form of daily subsistence or maybe even drudgery... not the same kind of fantasy-ideal that it seems to be for Jack. By the time we see the interior of Jack's house in Texas he's a salesman who works in an office and drives combines around a parking lot.
I wonder if the artificial signs of nature inside the Twist house (the fake cactus plant lamps) and the faux signs of being "western" in their decor might correspond a bit to the framed pictures of mountains in Ennis and Alma's homes (in the isolated ranch house and in the apartment over the laundromat).
The containment of the symbol of the mountain for Ennis may be slightly different in meaning from what you've pointed out here about Jack's daily-Texas environment.