I have several thoughts on that whole issue, Kishido.
1–I do think Akainu used some form of haki to make his punch more powerful--the reason I believe so is because he set out to punch Luffy–who had just been seen using CoC not too long before this scene, and as such was a relatively unknown quantity to Akainu in terms of how powerful he and his haki were, even if he guessed Luffy used it unconsciously--he did not go after Ace to begin with, so was prepared for the unknown.
2--I've toying with the idea there are stages of a logia's forms, similar to Chopper's stages, where they are partially in logia form and partially solid. That would explain why Ace was still flaming, but somehow solid at the same time--perhaps he had to go flame-on to get there on time to save Luffy, and hadn't had time to fully turn back to solid by the time he got fisted--or perhaps he was hoping that partial fire-form would be of some help--at that point he had decided to live, so he didn't want to die, and might have tried what he could, without risking Luffy.
3--It's just gross to think Akainu'd want to stick his arm through two people, one after the other, at the same time--not even a monster like Akainu could go that far, right? Right??? Not to mention that he may not have been able to see properly to aim for Luffy once Ace was skewered on his arm and effectively blocking his vision...