I read an interview with Larry McMurtry and Diana Ossana that they don't believe Ennis would have committed suicide. Here's what they said:
According to Proulx, Wyoming has the highest suicide rate in the country—and most of those suicides are elderly single men. Did you two imagine Ennis’s future at all? Ossana: I think Annie has a story somewhere about Ennis as an older man, and he winds up moving in with Alma Jr.
McMurtry: I don’t think Ennis would kill himself.
Ossana: He’s too tough. That would be a sign of weakness, and it would leave a memory of him as being weak, and I don’t think he would want that. But I do think that Ennis knows that people probably know that he’s homosexual, and emotionally [at the end of the film] I think he makes a tiny bit of progress, because he agrees to attend Alma Jr.’s wedding. Finally he compromises—
McMurtry: And doesn’t disappoint a woman.
Ossana: It’s the first time in the film that he doesn’t disappoint someone, male or female. It’s a tiny baby step, but he does it. I just don’t know how much [more] he’s capable of changing. I think if anything, he might become even more homophobic and bitter because of what he did, what he gave up, what he lost, what he’ll never have.
The rest of the interview is here:
http://www.advocate.com/currentstory1_w.asp?id=25277&page=1