Louisa is great, and I also prefer this version to
Luisa. I don't think it's too dated. I could definitely see
Louie,
Lucy, or
Lou as nicknames. And I don't think it's too matchy with
Laura, especially as
Laura wouldn't be a sibling's name.
From your list, I like
Louisa Emilia, even if it is a bit sing-songy. I guess I like
Louisa Emily, too. It seems like a tough name to pair mns to.