It does not mean they were fusing because he was a demon if he was one or something, that did not mean he was dying. It doesn’t matter which emotions he uses as long as he has emotions to power his flames it does not state it has to be magic or not, curses are just the thing for hatred and such. Gray was still alive and kicking, which meant he wasn’t dying at the moment. There is science to things which also apply to scaling from the consistant standpoint, which means things can be tweaked for the sake of story telling. Still does not put dragons at Spriggan level and that is a fact and story state Spriggans are the strongest enemies FT faced. And no, he was using a DKDF attack, Savage flames isn’t a specific technique just flames that are savage. Again, Zeref he did wanted Mard to succeed which he congratulated for the attempt. Zeref only guess with Igneel’s power, no proof
Natsu was like 85% human. Only his hands were demonic. Also, just because he gained demonic traits doesn't mean the seeds weren't fusing.
Yes, it matters what emotions he uses. This is the entire point of the series. Natsu specifically said his feelings towards the guild is what will give him his greatest power. Those
emotions are Natsu's strongest power.
The main point is Larcade was no more injured than Gray. Gray was injured and collapsed when his rage ran out.
Saying that things can be tweaked for the sake of storytelling for Bluenote, and then adamantly insisting that we stay strict to science for Wall....is a textbook example of cherry picking.
Destroying face absolutely puts the parent dragons above the Spriggans. None of the Spriggans can pull off a feat remotely close to this scale, except maybe Irene in her dragon form. Even then, that's by scaling not feats. If Ishgar had face, they would shut down the Alvarez Invasion easily.
Story never states Spriggans are the strongest enemies Fairy Tail ever faced. It was in the volume afterwards, but I digress. Obviously there are exceptions, like Acnologia. Parent dragons were not enemies. They were allies.
Actually, if you want to get technical the attack that burned time was actually a Fire Dragon King's Demolition Fist. The attack afterwards that took out Zeref was an unnamed punch. When Zeref said that Natsu's flames were a blaze of savage emotions, he wasn't referring to a specific attack. Just at the fact that the flames Natsu were using are powered by his savage
emotions. Those
emotions are referring to Natsu's feelings for the guild. Natsu can only use savage flames if he converts his feelings for the guild into power.
Actually, there is proof. That Igneel's power would have killed Zeref. It was in the 'story thus far' section back when the series was still doing weekly serialization. CR used to host these chapters, before removing everything post volume 30. Luckily, I have the text (and page) saved. The text tells us that Natsu and Zeref were indeed both about to die.