I don't know anything about the Kopechnes, other than that they were apparently unsophisticated nobodies from nowhere, and perhaps easily controlled by the Kennedy machine. Their only child obviously bought into the Kennedy myth, so they just might have consoled themselves, or been encouraged to believe, that they were honoring her memory.
The Moxleys (Martha Moxley, murdered by the nephew of Ethel Skakel Kennedy, wife of Bobby), were just as wealthy as the Skakels, and it still took them millions of dollars and over 20 years to bring the killer, Michael Skakel, to justice.
Even if they had wanted to, exactly what resources did the Kopechnes have to use against the Kennedys?
If you want to assign blame in this travesty of justice, you're better off pointing fingers at the local and state officials who bowed and scraped to Teddy, and thus enabled him to walk away from Mary Jo's death.
You were doing fine until your last sentence.