I am a little bit confused with the G-5 marines, as it was stated they have no order and presumably no discipline, they are more or less like Pirates heck they even look like them(I think they are the first marines we have ever seen acting this way), So why are they still functioning if Akainu is the Fleet admiral, I don't think he is the type that can condone such behavior from his men and especially leaving them in a Marine base, I don't know they don't seem to fit into what I pictured the Marines to be when we heard Akainu was fleet Admiral.
First, it takes time for a new leader to put his "stamp" on an entire army. Regardless of that, you are always going to have people that will not follow orders irregardless of who is in charge.
There are also some confusing things about Jinbei. Basically, unless I am wrong, he kind of "gave" FI to big Mum, and I don't get why, Fishman Island was whitebeards territory and unless they were disbanded the Whitebeard pirates should be defending his territories, but Jinbei kind of ignored this and asked Big Mum for help?? I don't know something just doesn't add up, and why did Blackbeard allow Big Mum to take such a valuable and strategic territory, since he has apparently conquered most of the others, why did he leave FI alone long enough for Jinbei to make his deal with Big Mum?
Isn't jinbei leaving with the straw hats and inevitably going up against Big mum kind of make him become an oath breaker? because I am assuming the candy thing was part of the agreement that Jinbei obviously consented to, so him claiming to be going against Big Mum to me seems kind of out of character.
Pure speculation, but I don't think the Whitebeard Pirates are strong enough to proclaim that entire areas in the New World are under their protection. Jinbe did what was best for Fishman Island. Big Mum is an extremely powerful pirate and a great candidate to carry on Whitebeard's role as protector of the island and therefore keep relative peace.
There isn't any important reasons why Blackbeard would care about Fishman Island.