Remember, you need to put everything in writing. It won't do any good to track down his tax preparer
; she can do even less than you.
You have to make sure you put in writing that you did not sign the return & if it is signed it is forged.
I don't even know why they are sending it to you. What are you supposed to do with it? They should be contacting your ex-husband and shaking him up; there's nothing you can do except tell them - in writing - that's very important.
Does you ex-husband know what's going on? Of course, one way to end this fiasco is for him to pay back the credit, "your" half first.
If you're still talking to him, you might tell him the IRS wants you to sign an affidavit (in blood), under the pains & penalties of perjury, swearing that you didn't sign the return and that if it was signed, your signature was forged.
Just for your information:
That's a criminal offense & he's going to find out what happens when the IRS assesses both civil and criminal (monetary) penalties. He'll wish he was in jail, by the time they get through with him.