Hoping to adopt my stepson

As of March 27 I was a non-custodial stepmother. We picked up my stepson that day from school with the remains of a bruise where the ex had choked him. We filed a Domestic Abuse Protection Order the following Monday (we refused to return him that Sunday). On April 9th we recieved a signed and dated letter from the ex stating that it is in my stepson's best interests for my husband to have custody, for her to terminate her parental rights and for me to adopt him. The following day she showed up at the DAPO hearing and didnt contest a single thing. That gave us temporary custody for 3 months, to end June 3oth. Child support has already been stopped. She is not paying child support. I am frustrated beyond belief.... I can not find anything dealing with steparent adoption where the steparent is (technically) non-custodial. Our attny has drew up a joint petition asking the Court to modify the original decree to give my husband custody. I am not sure that she is going to sign it... (long story). Im nervous on that... Do any of you have any experience with this in specific?