People said the "Obito=Tobi" theory was bogus because there was no way wimpy Obito could pull off all that shit and know so much,"it was impossible"-they said.Well,we all know how that little tale ended up getting confirmed.So let's not be so arrogant as to repeat it again here.We must consider all probable and implied scenarios,based on facts or implications from it.
When people do theories they usually ignore the nitpicky details that are still missing,like "how could Obito know about the legend of the rikudo and the ten tails?" .Those details ended up gettin explained AFTER the theory was confirmed.
Igneel did train him,but we don't know WHEN and HOW Igneel met Natsu and WHY he decided to train him in dragon slayer, a magic that as we all know by now, only brought destruction, ruin and Acnologia.
Also, "E.N.D is the most powerful demon"...I won't be surprised if Mashima on purpose is using this expression with a different meaning,like E.N.D being "potentially" Zeref's strongest demon.So him being teached by Igneel, supposing that Natsu Dragneel is E.N.D, actually makes sense. And would allow the dragon to teach the demon that magic is to protect his friends,not for random destruction.
