Now that I think of it the Michael Cohen situation was probably different. He was cooperating with the government against others (Trump) instead of just taking a plea deal, and that probably changes things. Generally a plea deal goes like this: Prosecutors indict the defendant for crimes A, B, and C. The plea deal might involve the defendant pleading guilty to A in exchange for the prosecution dropping B and C and recommending a reduced sentence. If they deal falls through they can try the defendant on all 3 crimes, which they have to prove to a jury to get a conviction of course. I think they just have to come clean if they're cooperating for a reduced sentence.

What makes this whole thing suspicious is that Hunter is still under investigation. The way it should work is that they complete the investigation and then decide what to charge the target with. Only then would a plea deal be negotiated. The fact that they're still investigating is pretty damning evidence that they just wanted to bail out Hunter and make the whole thing go away. That's where this weird ass agreement on future immunity comes from, and that's why I said that this wouldn't be an issue if they had done things by the book.