Wait are we saying that this version of Igneel is stronger than Acnologia? If so, then that settles the debate right there.
My estimate for how powerful Igneel was at his prime is basically Ignia. Ignia's flames can burn Natsu's hands, and that's a good indicator that it's probably hot enough to burn Fairy Heart. Zeref noted that time was burning a few seconds before his own hands started to burn, telling us that the temperatures required to burn his hands are just as high as those required to burn time. I honestly believe that Natsu with Ignia's flames would beat FH Zeref. I suppose you can argue that Igneel's magic power didn't make flames hot enough to burn Natsu's hands, but keep in mind that the power in Natsu's arm was only the leftovers after Igneel's fight with Acnologia.
Anyways, if Alvarez doesn't have Fairy Heart then Igneel stomps. Keep in mind that Acnologia wasn't just stronger than Alvarez Empire. He was stronger than humanity as a whole. Jacob can make everyone invisible, but Igneel can just drop a roar in their general area and hit them anyways. The historia of God Serena was weaker than original and oneshotted by Gildarts, so the chances of Neinhart bringing any historia that would pose a threat to Igneel is zero. Irene and God Serena being dragon slayers also doesn't really matter. Acnologia is way stronger as a dragon slayer than both of them combined, even factoring in enchantment.