One must remember that they were high up in the Rockies, (the WYO treeline is 9,000+ feet) where even during the summer months changes in the weather are often more severe than at lower elevations. It would not be especially uncommon to have the weather change so that one day it is quite chilly at night (especially if there is no cloud cover) and the next evening it would be warm and comfortable. I also agree that Jack probably bared himself purposely as a visual invitation and “come on” to Ennis, to get him into the tent. (It sure worked, huh?)
Also, the timeline between FNIT and SNIT has never really been nailed down I believe, and like so many aspects of the film, Ang seems to have left it purposely vague. Of course I think the terms FNIT and SNIT are inventions of the fans and do not necessarily mean day one, day two, etc. So could have been several days apart actually, whatever works for you.