I think Akainu is too irredeemable to be left alive at EOS. Maybe if Oda showed Akainu's flashback and the reason why he hates pirates I think that can make Akainu a bit irredeemable.
The most evil characters have to had suffered the most, in storytelling terms. And I think it's no accident how Oda drew Sakazuki and Teach as children. No doubt they will have the most heartbreaking stories. I don't think this justifies anything - other than there's a reason for every wrongdoing.
I think people have different opinion when it comes to Jinbei. I think what makes Jinbei one of the best because he helped Luffy on multiple occasions but it might also become the reason why Jinbei is boring to some people.
I think on Impel Down Ivankov and Mr 2 were the one who helped Luffy the most. Jinbei reason to be Impel Down was because he was opposed of fighting WB and WG put Jinbei there until the war vs WB pirates was over. Also the fact that Jinbei owed WB a favour and fought Ace till draw.
In MF I think most WB pirates were bust stopping Akainu. Buggy was too weak to save Luffy from Akainu. I think it also to higlight Jinbei's durability.
In Fishman Island I agree. Oda chose Jinbei to be the only one whose blood type matched Luffy's. For WCI SH was on the sea and to escape they had to use Thousand Sunny. Jinbei as fishman was perfect to help SH escape here since he could control the sea. I think this was also used by Oda to highlight Jinbei's helmsman skill since helmsman position in SH hasn't been filled.
For Ivankov and Mr 2 and Mr 3 AND Buggy it makes a lot of sense, it fit in with their characteristics and personalities. Mr 2 has always been very similar to Luffy in a way, and their time together made him sympathetic, so it makes perfect sense.
In Jinbore's case I think it was too forced. So... He fought so hard to save Luffy because... Luffy is good at turning everyone into allies? Because he owed Whitebeard a favour...? He helped a random dude who was saving his brother because he owed this other dude a favour? Or... Because he felt like it was right? And then later decided to comfort this random dude out of pity? I mean, is this guy even a pirate???????????? Then don't get me even started on all those things you mentioned later, he turned into Luffy's
dog fish.
I know the explanation to all of these ^, just trying to address the pointlessness of his character. I think he was particularly badly developed, and very unlikeable because he makes little sense. I was more convinced of Crocodile's help than of Jinbore's.
Just my opinion, of course, I'm glad you like him.