None of this precludes Unforgiven from being a western-noir. In fact, it's all quite in line with a noir.
However, I think you're oversimplifying the themes of Unforgiven by quite a bit. Munny wasn't righting a wrong done to a woman, he was trying to get money to help turn his life, and that of his children, around from his failing farm. He ends up "righting a wrong" but not one done to Delilah, rather that done to him and his friend, Ned Logan (Morgan Freeman's character), as well as confronting another not-so-nice-guy form of abuse in Little Bill (Gene Hackman). There are absolutely no heroes in this story. Munny does what he does for reasons we can understand, but Little Bill was not necessarily in the wrong in trying to protect his town and the towns people from men like Munny. The initial "seemingly trivial " event of the cowboy cutting up Delilah isn't trivial either, as it reflects all kinds of injustice on all kinds of levels, not the least of which was the retribution the whores were seeking for the wrong done to one of their own.
It's just a Western there, RobRoy. Leave it at that. What's this "noir" suffix's purpose anyway? To confuse? Unforgiven is a Western genre film. No noir. Unless you agree that the term "noir" has been overused to such an extent as to render it completely meaningless.
Yes - the initial event IS trivial - or didn't you watch the same movie I did? A prostitute impulsively laughs at the size of a man's johnson. And that is it. If she doesn't laugh - there is no domino effect. Otherwise he finishes, he pays, he leaves. End of story. But she DID laugh. That's seemingly trivial. Yet there is a very tiny barbed hook associated with that "giggle" (as Strawberry Alice called it). You never quite know how a man will react to that insult, even if it's unintentional or absent of malice. And this guy reacted - unluckily for him - angrily and impulsively.
Munny WAS involved in this - partly - to right this wrong done a woman. I think it was certainly a convenient justification. Money yes - but he could have just as easily robbed a bank or train or two. Observe the nonchalance of various characters as they discuss the retribution owed the woman for having her face cut up. The pimp isn't surprised that the girls are working to create a reward. Hackman isn't surprised. He's worried - but not surprised. It's Old West, and there are certain things you just can't do to a woman even if she's a whore. Including cutting her face. And of course we have that other dynamic working in the Old West - tall taling. The exaggeration of how badly she was cut up, how it got more serious each time retold. Well, we certainly know about tall taling. Wyatt Earp built his legend on that dynamic. So did dozens of others - Calamity Jane, Billy The Kid, Wild Bill Hickok, the Daltons, James gang, Cole-Younger gang and so forth.
What we're seeing in Unforgiven is the conflicting sides of people - their contradictory natures. We're like that, all of us. Nothing noir about this, from how I view folks. Munny's become a decent family man - but he's going to risk his gains by gunning for two cowboys who dishonored a woman. Little Bill wants peace and quiet in his town, he's building his house, won't have guns being toted about, doesn't want violence. But he nearly kicks two men to death, whips another to death and appears to enjoy these activities on certain levels. Similar to Munny - deep down, these two men really have NOT changed nearly enough. That tough gristle is still there, driving them. They've merely papered over their roughshod natures. English Bob changes from an arrogant showoff to a whimpering, griping and thoroughly beaten man. The biographer gets the real lowdown on these dangerous men at point-blank range, rather than via the tall taling of English Bob. Flowery arrogance out the window. And so forth.
Still - we're talking about a Western. Unless you're just translating "noir" to mean "dark"? So many films are dark that I suppose we should just hook "noir" onto the end of everything. But "film noir" - that's a distinctive genre with some hard boundaries. "Noiring" everything, to me, dilutes that impression left by a good "film noir" flickerino.