Concerning the Admirals discussion: I think it's fairly obvious, as of this chapter, that there is more than just Kizaru as an Admiral. Jinbe suggesting that the Marines were granted unprecedented power could actually mean that there are currently more than three Admirals. To make up for Kuzan leaving and Sakazuki getting promoted, perhaps there are now five Admirals. Three teams, two teams of two to cover the power gap left, and Kizaru as the lone wolf. As for no known current Vice Admirals getting a promotion as of Film Z, if we assume it is canon, the argument could easily be made that not all of the Vice Admirals were present at Marineford. We know Vergo, who has shown to be quite powerful, was introduced in Punk Hazard and wasn't shown on page at Marineford, if he was even there. It's very possible that some Vice Admirals remained in the New World to watch Shanks, Kaido, and Big Mom while the war was ongoing. In fact, we know the Marines were aware of Shanks intercepting Kaido, making this possiblity likely.
As for Oda making a mistake, it's possible, but I would say fairly unlikely. The biggest difference between his mistake with Zoro's bounty and Law speaking about Admirals in the plural is that the former had precedent for a mistake to occur. Zoro's original bounty was 60 million berries. His second bounty was 120 million berries. It would be quite simple to make a mistake by crossing these two bounties to form the error of 160 million berries. However, with the latter, Oda described Admirals in the plural, with an additional kanji character, as CCC explained. If Kizaru was the only Admiral, why would Law, who works with the World Government, not simply say "If you quit the Shichibukai, Kizaru will come after you"? It seems obvious, at this point, that Oda deliberately said Admirals to entice speculation.
Concerning Luffy being strong enough to take out one of the Yonko: Considering Oda has made a pretty big spectacle of how powerful the Straw Hats have become, especially Luffy, I think that Kaido is meant to be the Crocodile of the New World. He will be Luffy's first major fight, but, ultimately, he will be responsible for taking him down alone. The Grand Line started with Luffy's first bounty raise by taking out a Shichibukai. I think it's only fitting that Luffy's first bounty raise in the New World will be because he takes down one of the Yonko.