I forgot some things:
Usopp has a long nose because he is a liar. Being Kaku a carpenter and having those "painted like" eyes and square nose it is more clear that he is inspired by Pinocchio, so expect him to be a big liar (something like a double insider). Indeed, when his false nose disappears then it will mean that he has stopped lying, just like Pinocchio.
This would explain why the gorousei gave this important mission to the idiot of Spamdam without much hesitance (because he was going to be watched by an admiral-class officer). Also it was weird for the government to go capturing nico robin (an important thing to the WG) without support of the military branch.
This would also explain why the gorousei gave the ambitious Spamdam the responsibility to find an ancient weapon. The gorousei were looking like idiots to me letting Spamdam operate on this without much supervision, Kaku being Akainu would just put things on their place. And what about Aokiji giving a golden den den mushi to Spamdam? This would open a way for explanations, for example, and I don't really believe this: maybe the golden den den mushi triggers the signal for Akainu to start being himself, or something like that.
That said, I think this theory is bull. It would make no sense to reveal that Kaku is an Admiral at this point of the story.
After arabasta we got the introduction of Sengoku, Whitebeard, etc. After Skypiea we got a fight with Aokiji. What else for the end of this mega arc that meeting Akainu and knowing that he is pissed and against SH? Not everybody's objectives are interrupted after being beaten by the SH, remember Enel? Maybe Kaku will not be beaten or maybe he will be beaten but he won't surrender.
Being the first your argument could hold but i honestly don't think that Kaku (or Sakazuki…) will be worthy a promotion (speacially to Admiral) after Luffy&Cia mop the floor with him.
Remember Tashigi? She got promoted after everybody mopped the floor with her. That said, I don't really think that Akainu is going to get promoted in the real future, it was just something you might want to consider as it opens more possibilities, as I said.
Yeah, it'd be a little silly for him to be hanging around with CP9 for five years if he has that much authority.
"If you are gonna do the job: Do it carefully!!" Akainu.
Plus, I don't want us to fight the Admirals just yet
Maybe they are not going to fight him as an admiral, yet. He could restraint, he could be beaten with a bit of luck, he could get away to report headquarters, etc. And… "just yet"?, it is chapter 400, and every 100 chapters the rythm of the novel intensifies, get ready for a lot of mayor antagonists falling at the same time in the next arcs.
Or maybe he just has a weird face. Every single character that had a cameo had his face hidden
I agree, but I don't think this is a cameo. Flashback's are not places for cameos in Oda style, cause they are narratively dense passages in which everything is important and is told at a fast pace. And, come on, same cap, same mouth, same hair color (I think) at the same chapter in the story, in and out of the flashback, being Oda an specialist in doing every male character different. It's just suspicious to say the less. Remember Spamdam in the Franky flashback? Right after the flashback finished, he showed up. So maybe Akainu is gonna be there with us and maybe we won't know until the end of this arc, or maybe before that.
EDIT: I replaced "Kuina" with "Tashigi" cause I messed up