I think ambiguousity is the hallmark of BOTH film and SS, so if there are those who can reach the author's ultimate goal by believing Lureen was pregnant by someone other then Jack, why not?
I don't, however. As played by Anne Hathaway, Lureen became a bitter disappointed woman, ravaged by her relationship with Jack. He was the love of her life, and could not give her what she was willing to give to HIM.
In the ss, we are given a far-sparer report, indeed almost nothing...one reason Annie Proulx herself said film could be a more effective medium then print. Diana Ossana fleshed out that character for the film, not Annie, although Annie was kept informed and in tandem, step by step.
I may be wrong but deduce from AP's enthusiastic approval that she felt the character of Lureen was true to her own vision. After all, she WAS crazy about that interpretation of Lureen!