A plea deal usually covers all crimes. The person accepting the deal can’t refuse to answer questions about any and all criminal activity. So after the deal is accepted and signed Hunter would have spent days answering questions with the belief that he was required to.
Thus his Fifth Amendment wouldn’t apply if all criminal activity was covered. You can’t refuse to testify because it can’t be used against you. However when the prosecutor said that several other crimes would be open, the deal was shot down.
Since it was news to the defense that the other potential investigations were not covered. That was the trap. The Defense believed it was covering everything. The Prosecution knew it didn’t. As soon as the Defense saw it wasn’t they canceled the deal and pleaded not guilty.
Now the Prosecutors have a real problem. And it is a real problem. They have to get the other case ready for prosecution while fending off claims of prosecutorial misconduct for misleading the defense.
If Hunter had accepted the deal and it was approved and signed today. By next week or two at the outside he would be facing even more charges. That was the trap. He would have talked his way into prison thinking he was immune to prosecution because of the deal.
If anything. The prosecution of Hunter just got more problematic. All the stuff you accuse Smith of doing to Trump, the prosecutors apparently really did do to Hunter.