The movie's message is ruined

I understand where you're coming from, but it doesn't "overshadow" the "main message about the old west giving way to modern civilization", it "reinforces" it. Same as it's dramatized in Ferber's "Cimarron", it's the female who gradually moves the West from "ya better start packin' a hand gun" toward courts of law and civilization. Halley's choice of Rance over Tom is a parallel but personal way of illustrating the transformation and who was behind it. Fits right in, I think, and makes the message even more powerful (as you suggest by using the word "overshadows") than the bigger "events" do by themselves. . . Anyway, that's my take on it.

"Everyone KNEW" that Hallie was Tom's girl. But just because everyone knew it, it isn't necessarily so. Tom loves her. But does Hallie love him? Consider that she has a pool of men to choose from and that Tom is single, handsome, and wealthy. She is independant, practical, and intelligent. She cares for Tom, and knows if she marries Tom, her life will continue to be what it already is. She'll still be cooking and serving food to a housefull of men who will come to work for Tom as he builds his business. With Ranse, she knows she'll move away. She'll only be cooking and cleaning for one man and that one day perhaps she won't have to do the cooking and cleaning.

I never got the impression that she was head over heels in love with Tom or Ranse. I did get the impression that she cared for Ranse the way one cares for a small child or a half-starved kitten.
I got the impression that she likes and respects Tom. She knows that Tom doesn't need her. (She is wrong, but that's what she knows.) I think that she has been living with regret ever since.

At what point does Hallie find out that Ranse did not shoot Liberty?

Hallie was choosing the future over the past. It's the same reason why Marian, in Shane knows she can't leave with Shane, and stays with her husband Joe. The woman chooses the future - laws and civilization - over the freedom of symbolized in the West of the past.

We could have high times
if you'll abide

2 things you are missing.

One her feelings for Ranse are spontaneous, she can't control them. She never tells Tom she doesn't love him in fact she does based on how tore up she was over his death. But she never plotted to get in bed with Ranse in fact they have no relationship at all, just shared feelings. It's Tom that notices how she feels about Ranse she never rejects him. You act as if she was screwing Ranse behind Tom's back. In reality it was Tom who rejects her.

Secondly Tom had every opportunity to pop the question. He was so certain he wanted to marry Halle that he was building the addition onto the house but never got around to asking her. Once he asked and she accepted her feelings for Ranse would not have mattered she would have committed her life to Tom.

What is disappointing to me and many viewers i'm sure is the way Tom this strong successful confident man completely deflated himself after the events that occurred. You would think that he would be stronger than that and pick up and move on but the image we get of Tom is that he spent the rest of his life poor and lonely while Ranse became one of the most powerful men in the country just because of these events.

I would agree that the most universal message of the movie is overshadowed but I think it was John Wayne's great acting that caused the distortion. His raw, stark heartbreak was too disturbing to get past. I can't watch the movie again because he makes me cry every time. And I rarely cry watching a movie.

As I mentioned above, Tom was giving Hallie to Ranse, understanding very well the direction of the New West, in which he had no part. That's why he's shown as dead in the beginning, with no one remembering him. This is a trait of the Western Hero, a basic understanding of the forces of history. Again, as I wrote before, it's similar to Shane's leaving at the end, allowing Marian to stay with Joe.

We could have high times
if you'll abide

The movie's message isn't ruined by Hallie's "betrayal" of Tom in favor of Rance, it is rather made through it.

Ford wasn't simply trying to validate the course of history or the march of civilization's progress. He sees Hallie's choice of Rance over Tom as inevitable. The future must win out, it's simply the way the world works, but he wants us to realize that it's not a simple case of what's new is better, we must trade in the old and when we do there is something we lose. That's what Ford wants us to do. Reflect on what we lose. To not simply forget what came before. Progress may lead to growth and security, but we gain it at the cost of freedom and innocence. The film doesn't moralize and say "this is the way things should be", it simply shows us the way things are.

That Tom essentially forces his of Hallie (and the world in which he exists) by shooting Valance for her sake makes him a tragic figure, but it also gives him depth and nobility. He sacrifices himself so that she may grow up in a world fit for her. That is a gift greater than any Rance can bestow.

As a woman, this is what I see. I think she probably loved Tom. But the difference I see in the relationship between Tom and Rance is that Tom treated her as a pretty girl whom he wanted to marry, but he spoke down to her. Rance treated her as an equal. Tom really loved her and it broke his heart, but to be an independent woman in those times and be with a man that treated you as an equal... I can see the draw in that. You can't choose who you love.