Just pushed an update.
You can't restrain but you can damage their head or torso when downed so it takes them longer to recover. Go too far and kill them and it will have knock on consequences for that faction for the rest of the game. As part of this, I fixed the issue with uninteractable dead bodies.
Made some tweaks in dialog and what they say out loud to highlight that each faction won't work with each other to escape as they hate each other that much. You have to pick a side.
To balance this out, there are now 2 members of each faction as prisoners and I've increased their stats so you have a better chance of escaping with them.