My instinct here would be to (a) talk to the company who made the copyright complaint, so that you can tell Yahoo that the issue is resolved; and then (b) keep plugging away at Yahoo Customer Service by email until they put the site back up, or at least give you the domain back.

Large companies are highly litigation-sensitive. If there's any suspicion of a possibility of being taken to court, their easiest route is to just pull the plug. They won't even consider doing anything to help you until you can show them evidence that they are 110% safe from prosecution.

But I wouldn't give up.