1+4) It may connect to Acnologia's past, because dragons are still a mystery more or less. We don't know about entrusting Natsu but it seems like we will learn more in the future since now we going back from the beginning of Natsu and we pretty much may uncover the questions.What I'm finding funny is now this:
- Everyone in the manga who is a dark mage wants to bring Zeref back to life, but he never died. He's immortal. (Pretty much the last question left is why he was on Tenrou island asleep, but I suspect it's because he is the one who buried Mavis.)
- Every demon that Zeref created wants to summon END, but he's already been summoned somehow, and he's Natsu (or partially Natsu.)
We knew that Zeref knew Natsu from the S Rank exams arc, but we thought it was because of time travel that Zeref panicked and said. "no it's too soon, you can't kill me yet." At some point, END was supposed to become strong enough to kill Zeref.
This ALSO explains why Natsu is over 80 years old and could get trapped by Fried's runes during the Laxus Insurrection arc.
I still have some questions that need to get answered for the rest of this to fall into place, but this was a huge chunk of it
- Why are there dragonslayers? Why was Natsu entrusted to a ghost of a dragon, and by what magic?
- What is Lumen Histoire, and what is Zeref's connection to Fairy Tail aside being friends with the founders?
- Who summoned END originally - was it Mavis?
- Acknowlgia - how does he fit into all of this mess yet?
2) I don't know. As for Zeref and Mavis, we don't know the full extent of their relationship outside the fact they did meet each other and judging how they meet up once again in the current timeline, it seems like they ended in a bad note. I don't know if the Gaiden is a requirement to find out but we'll see.
3) We don't know who summon END. Maybe Zeref once did but we don't know completely.