https://www.backyardchickens.com/threads/mareks-survivor-reintegration-w-flock.1124794/
Read thorugh the thread, one of her lower down comments actually expalins what worked for her. Hope this helps
You didnt do anything wrong so do not worry about that, mareks is relatively common so do not blame yourself. I read a thread a while back where a woman shared her experiences in helping the recovery of her chickens i will do my best to find it for you.