Do you have a legal citation to support this? Every plea deal in a criminal case is signed under the threat of more serious charges, isn't it?
Couldn't the defendant who pleads guilty to manslaughter to avoid a murder charge say the same thing? "This was extortion. I was under duress when I signed that plea deal."
False analogy. If a defendant were to were allowed to withdraw a guilty plea to manslaughter in a plea agreement, then the prosecution would be allowed to return to the original charge of murder. In this case the DOJ does not want to bring any charges. A judges role is to mediate disputes between litigants. Here there is no dispute; they both want the case dismissed.
You have to have been charged with the crimes to accept a lesser plea. To threaten a family member with arrest to get a plea is simply extortion.