Next to Normal/WMG

Diana actually dies at the end.
In Light, Diana actually says "you don't have to be happy or alive" not "you don't have to be happier alive". Shortly after she abandons them, she commits suicide, making it even more of a Downer Ending. If the line is "you don't have to be happier alive", Alice Ripley pronounces it very oddly in the OBC, suggesting she also believes this theory.
 * At the risk of being a killjoy, the line is actually "You don't have to be happy at all/To be happy you're alive," which can certainly sound like "happy or alive" if you have terrible diction like Ripley does. But even the mondegreen version of the lyric isn't all that suicidal in context (although it does contradict itself - you don't have to be happy to be happy?).

Gabe is actually a ghost, and not a hallucination.
This'd explain why no amount of therapy will get rid of him.
 * And why Dan appears to see him toward the end of the show.
 * And why he can deliberately place drugs where his sister can see them.
 * He's really just a metaphor for shit going wrong in the family. In "I Am The One" he appears as frustrated as his father is (embodying even the sexual frustration, yes) and his mother imagines him as something that she wishes Dan to be, defending her and coming to her aid etc. It's pretty obvious in "Superboy and The Invisible Girl", too; Natalie is not actually being mocked by Gabe, she's just always haunted by this idea that her mother will always want Gabe and not her living child to the point where it could be right behind her and out of sight, laughing at her. You can see in "Catch Me I'm Falling" when Natalie reiterates "she's not here", because she's still thinking about it. Also in the Off-Broadway version when Gabe first comes to see Dr Madden with Diana and tells her that it's not safe and can cause brain damage. It's most likely a voice in the back of Diana's head that she wants someone to have to make her feel better or even stop her. There are other examples, too.

Next to Normal is a Darker and Edgier musicalization of The Middle.
Diana = Frankie Dan = Mike Gabe = Axl Natalie = Sue

Diana told Natalie she was leaving the family right after Maybe (Next to Normal).
This explains why Natalie goes right into helping her dad at the beginning of Light, and why she seems more resigned than surprised.