I agree that the early glances were not sexual at all. They were both there for a job. Were they going to compete for the same job? You check each other out. They're in the trailer and find out they're going to work together. Is he going to hold up his end? You check each other out. Then on the mountain, you look at the distance to see the other. It's loneliness and wondering what the other guy is doing. That Jack didn't even glance in Ennis direction while he was bathing or Ennis in Jack's direction while he was answering nature's call is important. They respected each others' privacy. To say it was immediate attraction detracts from the almost spiritual connection they had. They first had to trust each other to become friends. That trust and camaraderie made everything else possible. That first night in the tent came as a surprise to both of them it seemed to me. Yet it also seemed that Jack had been around that particular coffee pot before if you know what I mean.
I know exactly what you mean and I love the way you put it all together. Checking out the competition, the co-worker, the camaraderie, building up to the moment when Ennis talked for more than he did in a year...
Although I agree with the way you interpret the glances, I also think there is
some attraction or connection or whatever. Let's say they don't find each other repulsive.

I don't think FNIT was a surprise to Jack. I think Jack was pretty darn sober, jumping up to grab a blanket for Ennis as soon as he asked for one. I also think Ennis wasn't used to drinking so much, and Jack was sure not above taking advantage of the situation, if you get my drift. He might have been surprised by how fast Ennis sobered and caught on...

so he was definitely surprised by something.