Honestly I think that Umi will either commit suicide due to some internal pressure that she is putting on herself and not expressing or she will kill someone. I think Umi is silently a crazy person.
The way the current translation is worded makes it seem like he's neither her boyfriend nor her osananajimi/childhood friend.
[ul]
[li]Even if I'm not her boyfriend,/ even if I'm just a childhood friend. (parallel structure and repetition to get his desperation across) or[/li]
[li]I don't care if I'm not her boyfriend.../ ...and just a childhood friend. or[/li]
[li]Even if I'm not her boyfriend.../...and just a childhood friend.[/li]
[/ul]
You are my sunshine, my only sunshine
You make me happy when skies are grey
You'll never know dear how much I love you
Please don't take my sunshine away