DM is reporting that she will be attending, at the very least the evening party. That Harry has pushed the issue, insisting she attend.
Honestly I think if she got an invite she would be able to keep her mouth shut out of respect for her nephew. I think she has been felt so left out for so many years, watching her daughters from a distance, she would be happy to be part.
Mark Phillips has moved on better with his life IMO.
Sarah has clung on to her 10 years of life as The Duchess of York to the point of living with her ex-husband. Even Andrew still refers to her as The Duchess of York. I think that she's his "non negotiable."
Because she still is. Until the day she remarries she is Sarah, Duchess of York. Its not a title, it is her last name. Its what is proper to refer to her as.
Mark is not comparable. Mark was always a private citizen. He continued his equestrian career during their marriage. Life went on after they divorced. And their divorce was not civil, he moved on before they were divorced, as did Anne.
Fergie on the other hand spent years as a member of the royal family. Her job besides raising her daughters, was supporting her patronages. She wasn't going to go back to her jobs she had pre-marriage, and as a divorced former royal there aren't that many job options. Her and Andrew chose to remain in the same home when the girls were younger to give them one house and more stability. They wouldn't be the only divorced couple who eventually came back together.
Honestly if not for the cash scandal, and likely Philip, I think these two would have remarried years ago.