I start loving Irene more and more!! I don't know what she's about to do, but I hope it will be awesome! Everything else would disappoint me.
For the Gajeel/Levi part, he will surely come back. This was like "too sudden" for him to disappear, and of course after Levy started crying and...
Right, it's still too soon to be happy/mourn over Gajeel's death. Even if I can't say how he will be saved, he might be. But there is also a possibility he actually die, so lets wait for the whole chapter^^
I think it won't be that short. Irene won't be introduced as like the most powerful woman of the 12, just to lose against acnologia so fast.
But maybe the fight will be interrupted by the appearence of END, we don't know^^
I highly doubt Gajeel will die, these spoilers would be TOO obvious for that. And since we know Trollshima, something will happen so he will be saved. It would be sad that Gajeel will be killed by an enemy like Bradman, if he die he deserves a dead by someone much stronger than him :)