plot is not flawed, it is absurd

After Darby's near murder and she knew they were after her, she should have sent a letter to every newspaper, tv channel and reporter she could saying what happened and the rough details of the brief. The car exploding would have already made the news, thus corroborating her story.

Even if they originally thought she was crazy, if she were killed, everybody who got her letter would go over it again with a fine tooth comb. More likely though, at least some would have believed her to begin with at least enough to dig deeper. So this would completely remove any reason to murder her and would create a strong reason not to.

Mattiece's plan was nutty. Far too many things could go wrong in the years before (and if) the case made it to the Supremes and the Justices often vote in unexpected ways. Murdering them would make everybody look at what the two Justices agreed/disagreed on. And it's not remotely credible that the plot would remain hidden for years, nor is it credible they could murder everyone that figured it out.