In the end, though, it strikes me as presumptuous that a reader or viewer could claim to "know" a fictional character better than the creator(s) of that character.
I don't even consider the film in this discussion, so I am always referring to Ms. Proulx's original characters, but even if I did include the film, the birth mother is the birth mother no matter who may raise the child.
Well, Annie herself said that Heath "knew better than I how Ennis felt and thought." Heath is also a creator, of course, but before that he was a reader.
Art is a collaboration between artist and audience. Especially enigmatic works of art, like this story and film, which demand audience involvement. Why would an artist leave room for ambiguity if she wanted to impose only one rigid interpretation? I don't think it's like a crossword puzzle, where you put together the clues and come up with the "right" answer.
Deciding to interpret something by finding out what the creator says about it and then using that is a guide is as valid a way as any other, I guess, though it's still a choice. And even then, I'd be careful about basing a whole interpretation on a sentence from an interview, or even an essay. With an interview, you not only have to assume the subject said exactly what she thinks, but also count on her having been quoted correctly and thoroughly and in context. Right there, that's a big leap. Even in an essay, the creator may be using shorthand to make a point. For example, maybe Annie had heard a lot of people say they thought Ennis was a straight man who happened to fall in love with a man, and she wanted to correct that misconception -- or at least assert her own intentions!
-- so she said, no, Ennis is gay. Does that mean Ennis couldn't possibly have one iota of attraction to women? That he's completely at one end of the Kinsey scale? Not necessarily.
However, I happen to believe that he's at least close to one end of the Kinsey scale. That's based on the way Ennis behaves around Alma, Jack and Cassie.