I don't know the parameters of what's possible in your country (much less what the long story involving the foster care is), but what you've posted send Red Alerts going off everywhere for me--knowing the little I know. I would be investigating/re-investigating all possible avenues to block unsupervised access.
Is there no way for you to bring charges against her now, for what she did to you?
My heart feels tired and confused, thinking of your dilemma faced with a son over whose life you don't have total control--where your abuser is now permitted by the courts of your land to abuse him. ((((((( Foreign_Soul & son ))))))) * gentle care and blessings *
Roadie