First off stick to the same thread it makes it easier to understand you without going back and forth, and you edited last post and then started new one. Just hit reply at the bottom.
If CSEA already has the case, then they may have already filed for his income tax, there's nothing you can do. If she does not want arears forgiven, there is nothing you can do. She may be able to forgive the arears, but both of them will have to go to court, talk to an Att. to see if it is possible. I don't see why she would give up the money though.
Like I said before the $50 a week is a gift, unless she states otherwise in court.