I mentioned it to say a high tier like the Zoro from that scene already had that AoE. We already got a glimpse of Akainu's AoE with his opening attack on the Marineford bay, just like we saw Aokiji's freezing AoE dwarfing the island.
It doesn't matter what tier Zoro is, you're comparing incomparable characters, Akainu isn't a swordsman that cuts shit, and again, cutting something in half isn't even a fraction as impressive as vaporizing it, that's extremely basic.
Akainu's magma should be above King's and Kaido's. He is magma incarnate. Haki plays no part in vaporizing that horn.
So yes, Akainu definitely can vaporize it, regardless of direct power level comparisons with Kaido.
That's not how it works at all, Ace and Sabo have never shown blue flames like Sanji has, at best you can say the magma fruit has the
potential to be capable of it, but that's as relevant as saying Fujitora has the potential to create a black hole or bring down the moon.