Wouldn’t the IRS red flag us for the past 3 years on this 401k plan that was cashed out and do people go to jail for breaking their divorce decree?

You were supposed to AMEND, not APPEND your tax return. If you and your husband filed a joint return you would be responsible with him for taxes regardless of the divorce decree which does not bind the IRS. If the return is going to be amended you should try to avoid signing the return with an explanation that you don't want to create liability for what should be his tax responsibility. You need to have assistance from a tax attorney.

Generally, people do not go to jail for breaking a divorce decree, it's possible that the judge could find you in contempt of court and have you sit in jail overnight or for a couple days but breaking a divorce decree isn't a crime. If you and your ex cashed out his 401(k) and filed a joint return without showing the income. First, that isn't very bright when you cash out a retirement plan, the plan sponsor gives the IRS a notice of the amount, the IRS often doesn't react with great speed but eventually they will come around to get the money plus penalties plus interest. You have committed tax fraud, the IRS can charge you with a crime (intentionally filing a false tax return) and the punishment can be severe fines and/or prison time.

There is no need to go to court. There is a need to append your 2011 return to pay what you owe. If you don't and the IRS discovers it, you will have a huge penalty.

Afraid I do not quite understand your question, you should be seeing an attorney if you have not already engaged one for your divorce action. Generally, people do not go to jail for failure to comply with their divorce decree unless they are found in contempt of court.

I would hope the IRS would red flag your return. His failure to obey the decree does not excuse your failure.