I actually love [name_f]Emmeline[/name_f] [name_f]Grace[/name_f]! I don’t think it’s too long to be a double-barrelled name at all, and I think it flows beautifully. I also love that both [name_f]Emmeline[/name_f] and [name_f]Millie[/name_f] [name_f]Grace[/name_f] have family significance, so you have lovely reasons behind the different names.
I don’t think it’s weird for you to call her [name_f]Emmeline[/name_f], [name_f]Emmeline[/name_f] [name_f]Grace[/name_f] and [name_f]Millie[/name_f] [name_f]Grace[/name_f]. Nicknames often develop naturally within families, and it sounds like [name_f]Millie[/name_f] [name_f]Grace[/name_f] has become a special name for her at home and at church. [name_m]You[/name_m] can absolutely introduce her as [name_f]Emmeline[/name_f] at school and let her decide what she prefers as she gets older.
I also wouldn’t worry too much about not having settled on just one name! [name_m]You[/name_m] clearly love her full name, and there’s no need to add [name_f]Lina[/name_f] [name_f]Grace[/name_f] or change anything just because you’ve discovered another combination you love. [name_f]Emmeline[/name_f] [name_f]Grace[/name_f] is beautiful as it is, and [name_f]Millie[/name_f] [name_f]Grace[/name_f] is a lovely family nickname. [name_m]You[/name_m] don’t have to choose between them!