Quote:
if e-mail is already in use to sign up today and not yet opened, she could not use her e-mail again tomorrow to register?
Yes according to the rules she can't use it for tomorrow. If shes uses the same email again it will go agianst the magical quill challenge. Just make sure that you remember the password to her account that you made and let her validate it...
But I don't know if it can be used if its not yet validated. From my experience she can use it again... but I'm really not sure. All I know is that if an account is not validated you can't login right now on pottermore.com ^^,