Very nice, This theory is possible.
Let me add my thoughts to this theory.
I think END and Natsu are two personalities of same entity. And Natsu vs END’ll be the battle of will’s but not strength. As far as strength goes, I don’t think Natsu is strong enough to challenge Mard let alone END but Igneel is confident about Natsu being able to handel END. So, I my guess is that Igneel believes that Natsu’s feelings/will are strong enough to overcome his own darkness/Evil side of him(I.e. END).
2) Zeref’s statement: Zeref said that either Natsu or END’ll meet him which means that only one of them can meet him. So,If Natsu able to overcome END (the dark side of him)then Natsu’ll be the one to meet Zeref or If END takes over Natsu then END’ll be the one to meet Zeref.
3) Statement of Silver: END is a fire type daemon. I don’t think any kind of explanation is needed here.