Realistically he had 0 grounds. He made her mother a vegetable. Possibly even raped her with intent to impregnate. He tortured his own child (in theory) for 12 years. Then abandoned her leaving her to wander in the woods. Possible sexual assault on the child depending on what fan Theory is right. We know it happened in MK Ultra and that is what El is based off of.

Also the government who has record of all that was the one who gave her to Hopper. I can’t see any reason they would give her back. Or even trust Brener. He’s a ego maniac who almost destroyed the world. While I can see the government trying to poke at the upside-down more, I don’t see them bringing a mad man like him back. Brener will probably try to start his own thing which will cause waaaaay more problems.