This definitely must've been the site for Aokiji and Sakazuki's battle. The guy on the den den mushi did say it was really cold in one portion of the island. They also did say that the island was remote. What could be more far off than an island that isnt even supposed to be on the WG's scope of islands than somewhere they aren't allowed to be for a simple fight? I bet they just werent betting on the battle taking days to finish.
I have a feeling that Zoro is going to potentially fight a large majority of the battles in these upcoming arcs. With the presence of this samurai nation and the heavy amount of sword wielding pirates, i think zoro will gain fame very quickly towards his dream.
That last emblem cant be the yonkou or revolutionaries. I think that last emblem might represent the shichibukai. When they glanced at the warning sign it did have all WG affiliations. If it was revolutionary then robin would have noticed it. The yonkou are just 4 unbelievably strong pirates who aren't united so it cant be their seal either.
Random prediction: I bet Aokiji shows up next chapter to help w/the dragon. I have this suspicion that not only is he still on this island but that this dragon isnt really an enemy.