"You mean everything to me. ~~~ ~~~~ 난 너야 유나야 " "난 너야" means "I am you." / "유나야" sounds like meaning "You = I". These match well with ""You mean everything to me." "야" in "난 너야" is used for a statement of a fact but "야" in "유나야" is for getting someone's attention. However, because of the similarity mentioned above, "야" in "유나야" can be regarded similarly as in "난 너야". I think "You and Me" or something like that would be better for the English title.

It doesn't mean anything but just a name. I just read an article where the singer of the song answered about the title. "유나는 마음 속 상징적인 첫사랑의 의미로 쓰였다고 설명했다." (He explained that the name was used as a title, as the meaning of a first love) 유나 is a quite common female name in Korea. One of the very pretty names. So, the very literal translation of the title would be "Hey, Yuna." I think "My goddess" is the best possible translation.

i don't think so. i never seen 유나야 as other meaning except name