[Corvina]

Call me naive, but the group could simply exclude him and go on.
He may be the formal leader and main vocalist of the group, but that's simply because he's the oldest member...
Nissy is by far the most popular and successful and imo also the best singer of the whole group.
Afaik Urata Naoya is not that popular and also doesn't have a notable solo career as well in contrast to all the other members.
So while this is bad for the group, I don't see a problem in solving the issue.