I think it is no coincidence that we go from the “Tell you what …. the truth is …. sometimes I miss you so much I can hardly stand it” straight to the tent scene with Ennis’ arm around Jack. Jack is fully aware that Ennis’ arm is around him. IMO, they had just made love and fell asleep while Ennis was comforting and holding Jack.I am not sure that I understand the second part of your statement. Are you saying that Ennis has the troubled brow or Jack? Let me just put it this way …. Ennis’ arm around Jack is reminiscent of the “dozy embrace” flashback we see later. That memory is one of warmth and comfort. I think that is what Jack is feeling in the tent.
I think maybe when Ennis says "Jack, I swear," he is telling Jack that in a do-over, Jack wouldn't have had to say, "Damn you, Ennis."
(Someday I really am going to watch the movie and keep track of every time Jack swears. )