It will be a very long arc, the little details don't matter for now. To get a good perspective on how things might unfold, you should consider that whomever Akainu fights, it will definitely be in an isolated area with plenty space to initiate volcanoes/island level attacks. Nothing suggests Luffy will have to rush to fight Imu after Akainu is defeated. He won't 1v1 Imu too so no need to worry about powercreeps. Don't know where Teach stands in all this.
But yeah it's a stretch to think Sabo can beat an admiral, let alone the strongest one and the one who holds a natural elemental advantage to him. And no awakening will not help, it was never about which is hotter, magma consumes fire regardless of temperature.
Magma being hotter than fire is a huge mistranslation that has been corrected over the years.