Has it occurred to you that Col. William Ludlow isn’t real?
Technically, once you start with the Roman general Arturias and then deduct out all of the Lancelot and Guinevere myths, neither was King Arthur "real" (although I still think there's something to be said for the Excalibur and Merlin myth-seeds, but that's a subject for another discussion).
However, some Stories need to be told.
Colonel William Ludlow does not have to be precisely "real" any more than King Arthur or Allen Quartermain, for the purpose of storytelling. It is enough that he represents the Archetype of Individuality against the stultifying constraints of Obeisance.
(And yes, I am an unrepentant Jungian, and think that Freud was a cocaine-addled fraud. We can proceed from there, if you think it important. Best, OP)
In case you hadn't noticed, the Movie's name is, ahem, (please allow me to quote): "LEGENDS of the Fall."
Given that, by its very Title, the Screenplay claims pretensions to Mythos and indeed even the hallowed ground of Christian Mythos (which is, I agree with Lewis, a Myth which happens to be True), one should probably expect that this Movie will ground itself upon the Truth of Parable, rather than strictly-provable History.
Of course, I humbly await correction if I am in Error.
But I am reasonably certain that my interpretation is correct.
Best, OP