Well, young Ennis has ruler-straight hair, as opposed to grownup Ennis' curls. And the child cast as Alma Jr. (who says "bring fish home daddy, big ones" and "I need crayons") doesn't resemble the other Alma Jr. actresses — in fact, none of their eye colors match. So personally, I wouldn't take Bobby Jr's casting into much account.
We're talking about the backstory of a secondary character; it doesn't make sense from a story-structure angle for there to be an issue here.
I agree with all of this. I wouldn't put much weight on the appearance of the actors. Even in real life, children often look nothing like their biological parents. And there's no point in their being an elaborate backstory to a character that isn't explored further in the film.
And yet, I have seen credible arguments, in addition to the timeline, that Lureen was already pregnant. Let's see if I can remember some of them:
-- Lureen is very eager to get together with Jack in the bar, and even more eager to have sex with him on their first "date." She IS in a hurry! For good reason.
-- L.D. Newsome gives Jack the brushoff immediately after the baby is born and then (supposedly) would even pay him to get lost, even though being dumped like that would have hurt his daughter and her reputation. But Jack's done his job, saving Lureen's reputation from even worse damage. And remember in those pre-Roe v. Wade days how disastrous and out-of-wedlock birth could be.
-- "Isn't he the spittin image of his grandpa?" repeated twice for emphasis. A relieved acknowledgment -- or perhaps a cautionary reminder to all listening -- that the baby does NOT resemble anyone outside the family.
-- Jack is far less concerned than Ennis about the prospect of readily ditching his child in favor of the sweet life, even though it seems possible that such a move would mean never seeing the boy again.
-- Thanksgiving scene: "This is MY house and this is MY child." Jack reasserts his paternity. L.D. backs down quickly -- because they've all agreed to uphold, tacitly or openly, the story that Jack IS the father.
Now, I realize all of these have other perfectly reasonable explanations. Still ...