It would have had to been much better because, again, Zeref was much more impressed by it than he was by FDK roar.
Well my point is that Zeref didn't die to either Flames of Savage Emotions or Dragon Force. And yes, technically speaking Zeref's body got obliterated by DF. He still didn't die from that either. END's attack was stuffed by Erza, so I don't even think Zeref needs to tank it per se. I think he can hold out his hand and block it. Even if he wasn't immortal. I agree that END can probably fight on par with Zeref like base Natsu did. Maybe even better. That's not really going to kill Zeref though.
I suppose it depends on how much power you think Igneel left to Natsu. I consider the FDK finisher with Igneel's power to be a stronger attack than Acnologia's casual attack. In the case of Mercphobia's pillars, Gray could only freeze one of them temporarily before Mercphobia turned it back into water. He couldn't freeze Mercphobia's big attacks like the whirlpool. I am dubious on whether Gray can freeze IgNatsu's attacks in the first place, and even if he could I doubt it'll hold.
In what world was Natsu having doubts when he was lying in a coma? The seeds grew because Natsu was doubting his humanity, not whether or not he doubts his will to kill Zeref. That doubt began when Zeref told him he was END, and it didn't go away until he decided he wanted to be human again. The seeds were fusing the entire time. Choosing one seed would have caused the other to crumble away. I suppose you can argue that he didn't choose the demon seed in a conscious way and only gave into it's influence....but then that wouldn't stop the seeds from fusing anyways. He'd still be dying.
He was mildly more impressed initially, then the power of the dead got mentioned and he got his hopes up.
Yeah, which just lends credence to power not being what you need to kill Zeref.
So what? Acno's attack was stuffed by Jellal, it happens. As for Zeref blocking with hands, sure, maybe, he didn't do it to base Natsu but I don't see a problem with it, being able to block an attack doesn't mean that attack can't beat you.
Considering Igneel was on his deathbed, I don't rank the power left in Natsu particularly high, it's certainly not %50 Merc tier. As for Merc turning his pillar back to water, that's two spells on Mercs part, Gray was still able to freeze one of Merc's attacks and when he did turn it back it still lost all it's force. As for IgNatsu being able to turn his fire back into fire, sure, I guess, I don't really see the point though, Gray's not gonna need to use IgNatsu's fire as a ramp.
He was having some pretty interesting dreams as I recall. Natsu was literally a demon when he was END, and he didn't care, I don't see how there's doubt. But let's say you're right, none of this means that 1. END couldn't have evolved further and 2. that Natsu would die if it continued.
Like I told you, all symptoms of the fusion(aka the coma) had ceased when Natsu became END, he wasn't dying, as END Natsu could still fight without issue. There is nothing to suggest that he would turn back into a human without Erza and Lucy's influence, which mean there is nothing that would drop him back into the coma, he would be able to reach Zeref.
As for evolving further, well, I don't exactly buy the further evolution theory, but if I did, the fact that he was able to demonize to that point in the first place is confirmation that regardless of whether the seeds are fusing or not, Natsu can in fact become more of a demon.
You lost me. Zeref's plan was for Igneel to train Natsu to be strong enough to kill him. Not only does he not include END in his plan, but he actually told Natsu that he hopes he would destroy END the next time they meet during Tartaros. Zeref literally said that Natsu would have the choice of destroying END or not, that only one of them would face him, and he hopes that it would be Natsu. I mean, you could say that he wasn't looking for a fight....but he's still telling Natsu that he would have the choice of destroying END or not, and that he hopes he does. How was Natsu going to tap into END's power after that?
Alright, let me break it down.
Why did Zeref give
Natsu to Igneel? So that Igneel could make Natsu
powerful enough to kill him. Ok, now
what exactly is the point in doing that? No matter how strong Igneel made Natsu, Natsu would
never be able to kill Zeref, because
power isn't what you need to kill Zeref, you need something more.
So, In order for Zeref's plan to make sense, Zeref would have had to have known that Natsu,
even as a little boy, had the
means to take his immortal life and what would the means be? END. There was literally nothing else special about Natsu at the time. Little boy Natsu already had the qualification to kill Zeref, he just needed the strength, which would come from Igneel.
As for your second part about Zeref wanting Natsu to be himself at the end, just one of Zeref's many contradictions. Zeref wants to see and love Natsu, he also wants Natsu to suffer, he also want Natsu to kill him, he also wants to kill Natsu