Actually it isn't that weird and it might just be the perspective on the name of the magic. Dragon slaying magic came from dragons and apparently demon slaying magic from demons. At first that might appear contradictory, but if you look it from another perspective, it's actually quite logical: if most magic doesn't not work against a dragon or demon, how are you going to beat them? Let another dragon/demon beat them. It's the principle of fighting poison with poison. So if humans can't beat a dragon with their magic, but a dragon can beat a dragon with his magic, rationally speaking if a human learned that magic, he should be able to beat a dragon too. From a human perspective that would be 'dragon slaying magic', but from a dragon it's a bit weird to see it like that. At the very beginning of the series Happy also explained that Natsu's magic transformed his body into a similar constitution of that of a dragon. This is also implied with Acnologia who became a dragon while originally being a human, though that was (partially) explained by him bathing in dragonblood.
End probably. I can't recall that it was said that Zeref 'cleaned' everything he created. That people have his books, proves that even assuming he tried to clean everything, some things survived (this might include Deliora and Lullaby). Igneel failed at defeating End, so he might have just made the other demons.
