Every now and then, I wonder if Jack didn't take off his jacket in all 13 takes (wait... if I'm talking about takes, then I should probably say "Jake" rather than "Jack," huh?), so it really is a continuity error. As much as I like the idea of the layers of clothing representing layers of protection, I have trouble imagining exactly how Jack could fumble around to get his jacket on while Ennis is collapsing into sleep, and still end up spooned in front of Ennis like that. (I mean, the jacket's dropped behind where Ennis ends up sleeping, right? So Jack would have to climb across Ennis and then snuggle himself into position? It's easier to imagine both of them just collapsing.)
I love the content look on Jack's face, too. I can imagine him sleeping, but I also like the idea of Jack being half asleep or pretending to sleep, -- not wanting to move, wake Ennis, and bring the night to an end.