I would like to help you with your question. I can understand your concern for your son and your worries about your daughter-in-law. Her birth experience and upbringing certainly sound like she may have had a challenging time as a child.
You were right in seeing the blog as public material and it certainly seems that your heart was pure in wanting to inform your other daughter-in-law about the content, and then later suggesting a family meeting. I do wonder what your daughter-in-laws motivation was in writing the blog in the first place. Was she hurt? Was this way of processing her feelings? Did she want one or more of the ladies from church to read it and feel bad?
In any event, it would seem that the blog incident has come and gone and that your greater concern now is about your son and how his wife's behavior will impact him.
You ask if her behavior sounds like NPD. While it is impossible for me to make a diagnosis without interviewing your daughter-in-law, the information you have provided leads me to say that she does not have NPD. Rather, what appears to have happened was that her encounter with the ladies from church left her feeling empty and uncared for and she attempted to process these feelings by blogging. While the words in her blog were offensive to you and caused you some amount of emotional pain, she is the owner of the blog and has the right to post whatever she pleases. Should she have used some discretion and been more careful about the words she chose? Certainly that would have been your advice to her. And, yes, it is unfortunate that she chose to use such raw language to express herself.
The pain she felt after interacting with the women from church may be connected to how she has felt treated by other women in her life. As you point out, her mother died in childbirth, she had conflicts in school, she had a difficult relationship with her step-mother...all these suggest that she may feel insecure and mistrustful of other women....and that she may have difficulty letting other women into her life.
I do think she will benefit greatly from your caring, loving, and compassionate responses to her. She likely has never had a trusting, secure relationship with a mother figure. This is something you could offer to her.
As to your son, if he has concerns about the behavior of his wife, has difficulty understanding her actions, or is concerned about his own future, then I would encourage him to see a psychologist so that he can gain a better understanding of her personality and its impact on his life.
I see that you are offline, when you come online I will be notified.
I can certainly understand why you want to warn your son and help him be better prepared should her actions continue in the future. My fear is that he may misunderstand your intentions; seeing you as trying to create a wedge between he and his wife or offering him something that he does not want. He is already feeling the sting of her poor behavior and, as you say, wants peace at any cost. That suggests to me that he is less interested in understanding why she acts as she does, but rather how he can manage her actions and keep the peace. Presenting him with information about NPD, therefore, is giving him information he likely will not want.
It is unfortunate that her behavior has alienated good people. And from the statements you have made...this may continue in the future. Likely the best you can do here is to point out these situations to your son and ask him what he thinks of them. Because he is an adult and this is his wife, he will have to decide what he can and cannot tolerate. This situation is a good example of letting "life be the teacher"...there isn't much you can say here really...and if they loose friends, alienate good people, have broken relationships with family members...hopefully your son will pay attention to these things and take control of the situation.
By staying in the background...being that supportive mother-figure to her...you will have the opportunity to have a positive influence on their lives. In the long run, being seen in a positive light is far better for all of you.
I understand that it will be difficult to watch this all unfold. However, I do think that the best role you can play is to be that supportive ear for your son and that steady presence for your daughter in law.
I wish you the best...