Not really, certainly less of a difference than Dragon Force as you yourself have previously agreed as I recall. As for his regular attacks being stronger than FDK attacks, meh, Base Natsu can do more than Base IgNatsu did, though it took more time and effort.
No, he wasn't beaten, he was still trashed, he was also clearly relying on his immortality, something END doesn't have to worry about.
Base Natsu doesn't need to win that fight for me to consider END capable of killing Zeref, he just has to push it to high diff, which he did.
I'm not sure END would've got past Gray either, I still think he kills Zeref, but like you said, a separate discussion.
If that were true then END's health would've been declining since he beat Dimaria, it wasn't. END's transformation didn't just limit the fusions symptoms, it cleared them outright.
Sure... and I could make the argument that IgNatsu was weaker than Natsu too. Tbh the place i rank END isn't all that different from where I rank IgNatsu, so if you want to play that game we can play that game.
This is Zeref explaining his plan summed up, and in the order that he explained it: END=Natsu=the person who will kill me.
It doesn't take a huge stretch in logic for me to get END=The person who will kill me, is it? I don't think so, It's basic math. Then when you take into consideration that END killing Zeref has been hyped for ages and that Zeref's entire plan makes absolutely no sense if Natsu isn't END, and I feel pretty damn confident in saying that my theory is as close to canon as it can get.
As for Zeref intending to win the second fight, that was after Gray had left and his chance to trigger END had vanished. Before then he specifically stated that he wanted to draw out Natsu's greatest strength with Gray's death, with Natsu's anger and sadness, he wanted to feed Natsu's negative emotions, which in your words, is END.