I will have to see Jack's ashes and like you say Janice ashes can be from anywhere. There was a crematory near here down in Georgia where they were just scooping up any kind of ashes and giving them to people. It turned out that they were running behind in getting people cremated. Nobody knew the difference. As the story and move indicate people are left to form their own interpretations of Brokeback. My feeling is that Jack did not die and I felt that way the very first time I saw the movie. That is my gut feeling and it will be hard to change my mind about that.
Also, when I was researching the questions for Chrissi's quiz I came across several references to the lack of love between Jack and Ennis in the movie. The majority of these comments had to do with the first night in the tent scene and these comments spoke about the rough sex with no caring, loving, compassion, etc. However, I certainly did not see that during the second night in the tent scene as well as the motel scene. To me there was loving and caring in both scenes. Beyond that when did we ever see any sexual scenes between Ennis and Jack? How would we be able to judge what we did not see? We can only guess and I sure can see the love between the two. The reunion scene was no doubt rough, but to me that was an indication of how strong that love between Ennis and Jack really was. The comments I saw related to this lack of love were also from people who did not "get" the movie.