What should I expect at a temporary custody hearing called a 'return'?

If he's a perfect father, I think you should take a deep breath. It's not uncommon for parents to split kids 50-50. It may not have anything to do with him telling the GAL things, true or untrue. She may just be trying to facilitate a fair custody schedule. I note that you say he's an abuser, and due to that, I might be wrong, but it may just be the 50-50 thing.