My parents were able to successfully fight birth parents for custody in 2 of our 4 foster to adoptions. The other 2 were not contested. If you need more help, I can see if they remember what the arguments they used were, although it has been over 20 years since the last trial.
It would be great to know what the winning arguments were. I think we are going to hire a lawyer. It was really stressing me out to think I had to figure it all out on my own, especially with the system being so confusing.