Murder at Mansfield Park belongs to an entirely separate group, an homage or pastiche in the most positive sense of the word. Using Austen's novel as the basic structure of her own, Shepherd sets about replacing the personalities of some but not all of the characters, including their relationships and backgrounds. She plays out the results in a manner that feels seamless with Austen's. As a disclaimer, I am not an Austen scholar, so perhaps there are lapses and anachronisms I did not notice, but for me the overall reading experience was as if I had discovered a new Austen novel. Perhaps it is not her best, but it is consistent with her established work and delivers many of the same reading pleasures.
In this version, Fanny's mother has married a rich man, leaving Fanny as heiress to an immense fortune. (And then both parents die, so she does indeed grow up at Mansfield Park.) As a consequence, it is she who is given pre-eminence in the Bertram household, not Maria and Julia. Shepherd switched Edmund from the second son to the step-son of Mrs. Norris, whose character is every bit as manipulative, insensitive, and greedy as before. How much of Fanny's meekness and goodness arose from her dependent poverty, the constant reminders of her insignificance? And would she have become a different person if she had been pampered, given precedence, flattered and courted by everyone in sight? Shepherd proposes that in her world, at least, Fanny would have been molded by this upbringing into a spoiled, frustrated, and headstrong young woman. The author carries through this assumption without making fun of it, although there are playful references here and there to other Austen novels (Henry Crawford is now an estate "improver" and goes off to work on the estate of Mr. Robert Ferrars, for example).
More than this I will not reveal, except to say that I enjoyed Murder at Mansfield Park far more than any of the Austen mash-ups/take-offs/sequels/spoofs, etc., that I have read. Indeed, it's the only one I consider worth re-reading. It is an absorbing and thoughtful novel "in the style of Jane Austen," by a new writer who clearly has worked hard to combine original and established story elements in a smooth and balanced way.