Even better ending…

Can't work ... that'd mean she knew who Ned was 13 years before the film began (the yearbook was from 1968 -- Matty was four years older than Kathleen Turner at the time)

Besides, there's only so far you can stretch the "master planner" trope before it becomes ridiculous. This film falls safely within the realm of credulity -- she pegged Ned as a patsy, got into his orbit and "allowed herself to be seduced," and led him to believe the murder was his idea.

To have her seed the yearbook would mean she knew Ned would be her patsy seven years before he even finished law school

that would be a really stupid ending with nothing in movie to support it.