well no wonder he was acting like that. He was in a crappy situation before and was mauled to look like that. Then he was transported and had his little life flipped upside down. I would have been hovering at a 45 degree angle too.
I'm glad you didnt euthanize him. He doesnt sound like a gonner to me. Just keep babying him and make sure no one is picking on him.
He'll hopefully grow all of his fins back eventually too.
I dont know a lot about tangs, but there are a lot of fish out there with crazy personalities. Fish that get all butt hurt if you move a rock out of position and sulk in a corner for an entire day. (yes i'm talking about one of my fish)
So for some of these "intelligent fish" to act like that after a transport it totally normal. Some wont eat for a couple weeks after a move.
I'm pretty sure that you can use Malefix for saltwater. I would use some of that. Its awsome at healing that kind of damage and helps with stress too. Its not a medication so you can use it a lot and not have to hospitalize them