I don’t see how the charges can stick.
They don’t have her body, it was cremated in Africa. They probably don’t have the gun either.
Sure the insurance claims seem suspicious, but besides that, they have a claim from her friend - 3 years later.
I think they can’t be just making a case on that evidence alone. The Feds don’t usually charge someone unless they think it’s solid, that’s why they have some insane conviction rate like 97%.
My guess is that the unnamed mistress has perhaps flipped under federal questioning and is going to be a witness for the prosecution. They may not be able to prove she helped plan the murder, but since she married him and benefited from the insurance fraud, that was probably enough to threaten her with a stiff federal sentence if she didn’t point the finger at the husband.