I would think that you could get the court to enforce the divorce decree. It might not even be necessary to go to court. If it's clear that your ex violated the terms of the divorce, maybe your lawyer could talk to her lawyer and get her to cooperate. But this is a matter of state law, not federal tax law. And as you said, we are not lawyers here. Your first step should be to talk to your lawyer.
If your ex amends her tax return to remove the child that she should not have claimed, and she gives you a signed Form 8332, then you will be able to file your tax return, or file an amended return, and claim the child. (You will have to file by mail; you will not be able to e-file.)
You don't need just one form. You need a signed Form 8332 for every year that you are the noncustodial parent and you claim the child. The custodial parent can fill out one form for multiple years if she is willing to, but you still have to send a copy of that form with your tax return each year.
Problems with divorce decrees not conforming to tax law are common. Many divorce lawyers, and family court judges, don't know as much about tax law as they think they do. You might want to try to get your lawyer to work with a tax professional or a tax lawyer.
