What about the whole "Only Seth soothes her mood swings" situation? Sure it could be platonic, but I'm not buying it.
Yeah, that is the moment when I started meth (god, this joke will go terribly wrong at some point XD), when I thought about how their relationship could go in the future (I even posted a comment in a comment section, don't remember which site, about how I felt that was the start of young love), but... after that one, there wasn't really much.
I won't give any definitive "answer" to this because it is too soon. I don't think it was developed, but Melie's "theme" so to speak is a need for friendship, for intimacy, and intimacy may look "sexual"(I will use this word but I don't mean literally) but actually is a form of connection that she lacked, was limited, or downright refused to her.
Seth doesn't care about her "outbursts", he accepts her and that starts to connect to a deep feeling in her, which brings out love, it can be romantic or it can be "platonic".
Ocoho on the other hand I can say with pretty much certainty that she has a crush on Seth, she sees him as a friend but her reaction is more aligned to someone who is couritng the possibility of physical attraction, so to speak.
Personally... after all these years, I like the idea that Melie just wants to be a close friend better, because I think it aligns more with her "theme" of finding love in others and ultimately loving yourself, as she was really conditioned to not love herself with all the abuse and hardline culture she lived in.