#1) I think the songwriter might have thought in term of complementarity, relationship. So, it would be right. In terms of that is what she/he is to me. Perhaps a sense of belonging.
As in "It´s amazing how a person who was once a just a stranger, can suddenly be the world to you." We could think that he is personifying "his/her light" and that the other person is the shadows to it ( his/her light.)
As for question #2), let´s think this is a whole set of words in a sentece:
"You fade away afraid our aim is out of sight"
Você desaparece, temendo que nosso objetivo principal está fora de nossas vistas (inalcançável talvez, se não literalmente falado.)
Anyway, and then the world of the lyrics runs wild, it could also be "Você some, temendo que o nosso alvo esteja fora de mira.", since he refers to Atlantis (the undersea "lost city"). So, the lyrics could be from a submarine point of view, I mean a war vessel. So, the aim being out out of sight might be a dangerou position to a sub, he should evade as to avoid being surprised by the hidden enemy.
Finally, the songwriter might have, by design, used such wording to effect, so one would do what we are doing. Try to guess what he was thinking at the time of writing it!