I guess Lureen didn't suspect about Jack and Ennis or Jack and Randall. She was too busy with money and business, although she liked Jack.
She thinks Ennis is just her husband's fishing or hunting buddy in Wyoming, where Jack was raised and probably Ennis was a childhood friend of Jack.
She really wouldn't say all those things about Brokeback Mountain and the ashes if she suspect.
And the story she told about Jack's death, I really think Lureen wouldn't tell such a detailed story like that for someone she doesn't even know, if that story wasn't true. If she just wanted to lie or hide something, she would invent some short and simple story.
They found Jack already dead, nobody saw anything... why would they create a fake accident? They probably found Jack dead next to the truck and the rim of the tire next to him.
That's why I think Lureen knew nothing and Jack really died at the truck accident.
Hmm...I agree with what you said about her not knowing when Jack was alive...I think she was too focused on her business, and not very focused enough on her marriage to realize that it was a sham. Also, I guess Jack was a lot more careful about hiding it than Ennis, ironically. I think Lureen was smart enough to figure it out if Jack gave any hints, but it doesn't look like he did. Perhaps Lureen's father suspected that Jack was gay. I say this because of what he said during the Thanksgiving scene with their son Bobby. "You want your son to be a man, don't you?"...it seemed like a jab at Jack, and Lureen's father stopped just short of calling him a queer right there. Perhaps Lureen's father tried to discuss with Lureen his suspicions about Jack, but she just brushed it off and didn't really think about it. I also think about the beginning when Jack was more obvious...before he met Lureen. They lived in a small town and those guys at the bar when Jack flirted with the rodeo clown, they obviously knew then that he is gay. So maybe they talked about it to other people, and that is how Lureen's father would have found out early on. I do believe he suspected.
However, I disagree with the other part. I believe she found out the truth about Jack after his death, which I also believe he was murdered, and Lureen knew. There are many hints that the story she gives Ennis is false. The tone of her voice, how the story sounds like she'd practiced it, and also the biggest clue of all is how she starts to fight back tears at the end when she finally realizes who Ennis is. I think the cops told Lureen that Jack was beaten to death in a hate crime, and that is when she finally found out. She felt embarassed that her marriage was a sham the whole time, and so she made up the story about the accident. When Ennis first calls her, she did not know who he was. I don't think she ever really thought he existed! All she knew was that Jack had a friend he would go visit a few times a year, a friend in Wyoming. She knew that Jack was born in Wyoming, and that his parents lived there, and remember what she tells Ennis when he tells her about Brokeback Mountain? "Oh, so it's a real place? I thought it was imaginary, where bluebirds sing and whiskey springs." Over the years, she realized Jack was a big dreamer, so she doubted if this place was even real, and if Ennis was even real either. Maybe she believed that every time Jack told her he was going to visit his friend, he was actually just going to visit his parents. But then Jack was murdered, and the police told her, and so she found out that he was gay...then when Ennis calls she figures it out.
She realizes that Ennis is Jack's lover...the one he's been seeing all those years...the real love of his life. That is why you can see the tears starting, and she's trying to hide her anger and pain. As for why she made up the story...I do think it was a story she made up for all her friends and people in her town to hear, and when Ennis called, not knowing who he was at first (or thinking he was just another friend of Jack's), told him the same contrived story. When she realizes that he was Jack's lover at the end, she probably felt bad about lying to him but also maybe anger towards him...because he was the person Jack loved and not her, her marriage was a lie. And the police would know if he died from a tire rim or being beaten, because injuries from a deliberate beating would obviously be different from a tire rim accidentally hitting him in the face.