I think you could call her that for sure, and see if others pick it up, but I do think “Emmeline Grace” is a lot to go by all the time and that a lot of people will drop [name_f]Grace[/name_f]? As I say, that shouldn’t stop you using it 
I think I’d move away from [name_f]Millie[/name_f] [name_f]Grace[/name_f] as a nickname if you don’t like [name_f]Millie[/name_f]. Do you like [name_f]Emmy[/name_f]? [name_f]Emmy[/name_f] [name_f]Grace[/name_f]? [name_f]Lina[/name_f]? [name_f]Lina[/name_f] [name_f]Grace[/name_f]? [name_f]Emma[/name_f] [name_f]Grace[/name_f]?
Or would you rather she was [name_f]Emmeline[/name_f] if not [name_f]Emmeline[/name_f] [name_f]Grace[/name_f]?