You've been through hell like I have. Losing my son to the blood transfusion issue. My oldest son was suicidial after his brother died it was a horrible time and I thought I was going to lose him too. He did dangerous things like excessive speeds and wrecking his car, drinking till he passed out. I left the witnesses after Dak his brother died 4 months later. For us it was a turning point, but even then it was nearly 2 years before my oldest son stopped his dangerous behavior. It did gradually decrease, but now he tends to close himself off from love as it hurts to lose people we love. He still in pain but he knows I'm here. Could he still committ suicide, yes because he knows it would stop the pain he feels in his heart. I just hope that with my help and his own healing from the religion he can find a way to live life the best he can and be happy. Suicide is about stopping the pain within.
Your nephew was in a dark place, and he made choices that you had no control over. Would he have changed if you had left the witnesses sooner? Maybe, maybe not. I know I still feel the blame for my son dying, for being so stupid about blood and believing that God would reject us for saving Dak's life if it would have. I know that we can learn from our mistakes, and the important thing is to reach out to other young people now who are struggling with rejection from their JW families as they try to get free of the JW lifestyle. We can help them, reach out to them and let them know we can help. We were blinded by a ignorant religion that didn't really preach love, but preached hate and judgement.
Hope you can find peace,