I don't think the decree is valid--just a personal opinion without doing the research. That being said, unless you decide to contest its apparent validity by filing a suit for declaratory judgment, it's quite likely the nunc pro tunc will stand. Why not spend a little money and pay for some detailed research?This question does NOT call for an "off the cuff" answer.
I am not intending this to be legal advice, because I don't know the particulars of your situation. Call me if you would like to discuss this or other isues.
Sign up to receive a 10-part series of useful information and legal advice about the divorce process.