Igneel doesn't name his attacks. To him, they're just
his flames. Igneel is the Fire Dragon King, so his flames
literally are Fire Dragon King flames. Every time Natsu uses FDK, he is using Igneel's flames. Natsu can use Igneel's flames with his own magic power in the exact same way he can use Laxus' lightning with his own magic power. Natsu isn't using Laxus' magic power every time he uses LFD.
We know that Natsu couldn't have possibly used Igneel's magic power against Ikusatsunagi, because he outright stated in no uncertain terms that he could only use the power once. That one time was against Zeref. Natsu actually still had enough power left for one more attack before Happy stopped him. If Natsu could save and portion Igneel's power, then he would have just saved that remaining bit. He couldn't, and the fact that he couldn't means that he never used it prior to Zeref.
Not only did Natsu not use Igneel's magic power against Ikusatsunagi, but it doesn't even matter if he did because the power of the dead was never stated to be effective against Gods. It simply has a chance of killing someone who is immortal. Zeref is not a god. He was simply cursed to be immortal by a god. Zeref is not affected by God Slaying magic. God Slaying magic isn't even effective at preventing his Ankhseram curse from killing you. Remember Zancrow? Most importantly, you don't need God Slayer magic to beat up Zeref in a fight. Natsu beat FH Zeref without God Slaying magic or Igneel's power. Ultear actually knocked out Zeref back when his Ankhseram's curse was the most active in releasing death waves. I'll repeat what I said in the beginning: There have been 2 gods in the series, and both have been beaten without god slaying magic. If you want to add Zeref to the list of gods, then that just makes 3 gods that have been beaten without god slaying magic.
END's power was never stated to be hax to overcome a god's power. END's power could move through Dimaria's time because it had more power than her god soul takeover. Dimaria's exact words was "How could he move through my stopped time? How could he have more power than god?". Even if you believe END has a hax that allows him specifically to move through stopped time, that still doesn't mean that END's power is super effective in damaging gods in the same way God Slaying magic is.