It could be Shiki, but didn't he have some wooden wheel crushed into his head? If not, then it could be another Roger pirate. The one with the axe maybe? I don't know his name. He had long hair with sunglasses, and it may have gone white after all of these years.
I'm guessing Luffy defeated Jinbe either by force or showing him in some way that he was the more dominant one, but let him go. If not, the crew together may have convinced Luffy to let Jinbe go, with them arranging another plan in mind.
As for the NFP excluding Hody, they may be defeated right now by the three brothers, only to be defeated by Hody later. My guess is that the defeated NFP will take more of those drug energy pills and undergo a similar transformation so that they can challenge the Strawhats.
Dekken seems like he wants to destroy the island. He must know he can't defeat Luffy otherwise.