I have come to see suicide as simply one doorway into death--it's no more, or less, moral than dying from a heart attack, succumbing to cancer, being killed in an accident or disaster, or any other way that people can and do pass out of this earthly existence (and which we all must, and will, do in our turn, someday). What I'm trying to suggest here is that there need be no guilt felt by those left behind by a suicide, and it is certainly not for me to judge someone who chooses that route. All I can guess at is that, at the moment it happened, a suicide seemed like the best option for the person who committed it, and I will only bless them and wish them peace. It is also instructive to reflect on the myriad ways that we, both individually and as a civilization, are arguably committing suicide, though our behavior may not widely be openly acknowledged as suicidal.
Proulx's last line is the fulcrum by which this issue, to my mind, revolves..."if you can't fix it, you gotta stand it." Suicide is a form of fixing, the kind that transpires when someone can't stand it anymore. If Ennis ever got to that point, suicide may have occurred. But the story both opens and ends with him, grief-stricken though he is, standing it. All else can only remain speculation, and is another way in which the author forces us to finish the story in our own lives, in our own hearts.