The penalty would be the terms of the agreement, and as I mentioned, there are mandatory terms. They have to pay a fine, a financial penalty. They have to forfeit any profits that they have obtained and disgorge any benefits they have as a result of their conduct. They would have to make reparations to victims or explain why they can't do that. They would have to pay a victim surcharge. Especially if compliance monitoring is involved, they would have to pay for that, which can be very significant financially. They would have to make a sincere effort to comply.
First of all, it's up to the prosecutor to determine, in their absolute discretion, whether to offer to negotiate. They have to be convinced that it's in the public interest, and the court has to be convinced that it's in the public interest. Also, partway through, either party can withdraw from negotiations, or the prosecutor can apply for termination. If they don't believe the compliance and remediative effect is being achieved, they can terminate and resume the prosecution. There's the threat of that, too, out there until it's finished.