Question: Why is Decker wanted by the police? When the police came to visit the ranch, Decker notices them, tips his hat and walks away. The police have a sketch of Decker that says he is WANTED.
Answer: I don't think it's ever mentioned. The Ludlows almost certainly already know, and in the scene mentioned, the cops won't say.
Question: Before committing suicide, why did Susannah cut her hair? I've seen this behaviour in other films but have never known the psychology behind it.
Answer: Cutting locks of hair is often done in memory of the deceased. Knowing of her impending death, she cut two locks so as not to have them tainted by blood, but dropped one on the floor. I imagine it was the one for Alfred, but it is just my opinion.
I have goosebumps with this interpretation. I always thought that it was related someone with "scalping." She kills herself. She is her own victim. I don't know.
Answer: I think Susannah cut two locks of hair, one for her husband Alfred and the other for her true love Tristan. They were mementos in some respect, but also a clue for her suicide - being married to one brother while loving the other who no longer wanted to be with her. Susannah was torn apart, in pain over losing Tristan and probably felt she could not be true or faithful to Alfred if/when Tristan came around again. She would also have guilt over violating her marriage vows if she were to be with Tristan again while married to Alfred.
Chosen answer: To give a definitive answer would be misleading because there's no one specific reason why someone does this type of thing. People considering suicide often start exhibiting odd and/or uncharacteristic behavior such as suddenly cutting one's hair. In Susannah's case, it could be a form of self-mutilation, an attempt to change who she is by altering her appearance, or it is a way of controlling something in her life while other events spiral out-of-control, and so on.
Answer: I always thought it was she wanted people to know. She didn't want the wound to be covered.
Question: Who is One Stab telling the story to? I thought maybe it was a relative of Tristan (son/daughter or grandson/granddaughter) because he hands the person the family letter which makes me think it WAS a family member, but who is it?
Answer: There's nothing to tell us who the listener is. Your theory that it's probably a family member seems entirely reasonable, but there's nothing in the film that would allow the identity of the listener to be narrowed down any further than that.
Question: What happened to Susanna's dog Fin she first arrived off the train with? (00:09:22)
Answer: It isn't uncommon in movie and TV series/shows for pets to be introduced then quietly or gradually phased out with no mention of what happened to them. This seems to be the case with Finn. After Susannah and Finn first appeared, Finn was shown four times (first when he went onto "Mother's room" and fourth/last lying near the tennis court net @ 00:19:08). "Many, many" years passed until Tristan returned home, so it can be assumed that Finn was no longer alive. Perhaps something tragic happened to Finn, but it might be best to assume Finn lived a long happy life and died of old age.
Answer: Very early in the story when one stab is narrating, he's kind of introducing everybody and makes a reference about Decker having "illegally" married an Indian woman, and there are several references to the fact that the father was not happy working for a government that was killing Indians and also went out of his way to make sure they were in no way discriminated against. I think it's possible that may be why they were looking for Decker and why Colonel went out of his way to hide his living there.
I believe Decker had been a bootlegger. That would be the reason why the O'Banions (sellers of booze themselves), were with the sheriff when they came looking for Decker at the Ludlow's ranch. And also, the reason why the O'Banions didn't want to say what Decker was wanted for because.it was their own personal vendetta concerning bootleg whiskey. Maybe they had reason to believe that he was still selling bootleg whiskey around those parts. Also, later in the movie, when Tristan returns home after 7 years, Decker says to him "There's good money in bootlegging if you know what you're doing." And, Decker says that with a huge smile. Like he definitely knew what he was talking about and could definitely give Tristan some pointers about being a bootlegger.
Alcohol wasn't illegal until well after World War 1. The initial scene with the O'Bannons takes place before the boys go off to war, and it's stated afterwards that Alfred voted for the Volstead Act making alcohol illegal.
Not sure about this. At the time the sheriff comes looking for Decker, it's roughly 1915, and liquor is not yet illegal.