The more I think about Lureen the more I love her (maybe I should pay a visit to one of Roland's threads... haven't been over to an "I love everything..." thread for a while).
She was smart. Her cynical demeanor towards the end was probably a coping/ defense mechanism, but she never behaved like a victim. And, like kelda_shelton said, she was far more worldly than Alma. We know that Lureen went to college, was bold enough to become a rodeo participant, was bold enough to make all the first moves with Jack and she could clearly hold her own in business negotiations with (I'll assume) a bunch of men in Texas who probably weren't used to dealing with tough businesswomen. Alma irritates me, for a number of reasons, one of which is that she does behave like a victim. And of course she becomes spiteful. Lureen is an outsider in her own right... she's a business woman and not content with the stay-at-home role that probably predominated in her Childress community. And, she and Jack are clearly pretty good friends. The knowing looks they give each other once in a while in the film suggest an unspoken bond... of a best-friend sort.
I think she did know about Jack but, that we don't realize this for sure until the phone call. (Of course there is lingering ambiguity... but I truly sense that she knew). And, I agree that her suggestion that Ennis go to visit Jack's parents and deal with the ashes was an enormous kindness. You can almost see the wheels turning in her head as she decides to make this gesture. I think she deserves enormous credit for this.