Hmm, I'd say you can somewhat justify him looking different, the new gangplank doesn't seem like a captain at all, which is somewhat fitting. I'm guessing the new skin is a few years after the current events in Bilgewater, given that he has a shiny new metal arm and looks a lot more ragged. Its likely at this point he isn't even a captain anymore, he's just a man with no crew or followers to speak of. I think this kind of makes sense because like you said Gangplank is smart, he wouldn't go immediately try and reclaim his titles, Bilgewater is to cutthroat for such a thing to work, he's a feared man with no allies and no followers, in a world like Bilgewater the only place that'll leave you is dead in a gutter with your throat slit. There's a reason he paid the doc for secrecy, if anybody found out he was alive they're more likely to try and kill him to make a name for themselves then to help him, he literally has nothing left to his name. He'd also have to deal with Miss Fortune, who has an entire crew and powerful ship behind her. Gangplank would have to go into hiding if he wanted to survive after the explosion.