1) Since when can August and Irene tiers hurt Acnologia? Even with dragon slaying magic, they would barely do any damage to Acnologia. Natsu was able to negate most of the damage from August's melt nuke to the point that Happy, Lucy, and Mest got off with minor injuries. Acnologia would do the same to any incoming magical attack. He tanked roars from Igneel without a scratch. It takes 20 minutes of brawling for Igneel to damage Acnologia.
2) Erza didn't hurt ROT Acnologia. Her swords shattered upon impact against Acnologia's scales. It did no real damage. All she could do was push him towards a boat, which is basically the same as Christina pushing Acnologia into the rift. This is not even an issue against some of the god dragons. Aldoron is way too big to be moved, and Mercphobia is way larger than Acnologia too (though he can possibly be moved).
3) Aldoron murked Gajeel in less than 3 minutes, then began soloing the entire guild with thicket.
4) Whether or not Fairy Sphere nerfed Acnologia is irrelevant. At best, we know that Natsu with 7DS can beat a human Acno whose paralyzed. None of the god dragons are ever going to be in that position. They're going to be fighting in their dragon forms, which nobody is oneshotting. And honestly, it wasn't just the 7 dragons power that Natsu had. There was also a massive amount of POF. Lucy literally connected everyone's feelings and then sent it to Natsu.
August and Irene can't hurt Acnologia, Erza didn't hurt ROT Acno, Giant Gajeel got wrecked by Aldoron, and Natsu oneshotting Acnologia's paralyzed spirit doesn't mean he can do the same to his moving dragon form. Also, the combined power of all the Spriggans aren't equal to Acnologia. That's why Fairy Heart was necessary. They don't get past Aldoron. The seed that impaled Gajeel is the size of a mountain, and at full power Aldoron is too fast for Natsu tiers to even perceive. Wolfen Seed can make an army of Zerefs. So long as they stay away from the main body, there's no issue with the curse.