My opinion used to change back and forth on this. But with time, it stays more and more on the accident side. Therefore I voted "I think it was an accident, but can't say for sure" and for "We are not supposed to know".
It's the same sentence by Annie Proulx that others have already cited, "Ennis didn't know about the accident for months...", which makes my opinion. This is not in Ennis's mind, it's not from his POV, it's the narrator's perspective.
Same as in the beginning (for example), when the story says Ennis was raised near Sage and Jack in Lightnig Flat - we don't doubt this, do we? And we don't doubt that Jack was buckteethed, Ennis had an high-arched nose and the last time they met was in May 1983. So why should we doubt the accident?
All indexes which point to murder are from Ennis's POV.
The movie I think is a bit different. I think the movie is more ambigious. The actual scene Ang Lee shows us, where Jack is beaten, makes such a huge impact it's hard to shake off. I remember when I first saw the movie, I thought Lureen tells us a story, but what we see is the not debatable, objective truth. Only later, when I was at home, I began to question it. Judging only from the movie, I'd say I simply don't know which version is true.