Animal control or the Humane society always does nothing. I have never gotten them to care one ounce for a barn full of slipper toed, skinny horses who had been standing in their own feces for years, or a tb that was so underweight I was surprised he was still standing. No horse case that I have called about have ever gotten any results from any authority.
The one thing that did change was a pets store I called about who had sickly reptiles for sale and water turtles without any water. Thats all.
So it doesn't surprise me that the authorities just shrugged and walked away.
As for the rescue, I'm surprised you got a rescue to work with you. It's hard, since they are so packed. But this rescue needs to step up and take responsibility for the case that they want to work. You aren't part of the rescue, you don't work for the rescue. Don't do anything for the rescue. It will get you into trouble. You did your part in getting the rescue in. Now it's time to back off and let them work. If they come to get the horse, don't touch it or load it. You want none of the legal poop that can and will hit the fan in cases like this.
You did your part, now it's time to do theirs.
Rescues have an amazing ability to persuade authorities into getting them to work with them. They also have amazing abilities in getting the owners to sign over a horse without getting a court order. They have a good push and pull.
You did what you were obligated to do. Now just step back and let the ball roll on course.