Hey gdaustin,
I am going to be honest with you, most of what you are requesting is possible, however it's going to take a significant amount of poking and prodding to separate things. Let me ask you this before providing any further irrelevant data on my end, photos, are probably one of the most important thing for any person when it comes to their digital lives, are you guys using iCloud Photo Library on your devices and sharing photos, like are your library identical at this point or are they different, if they are identical, a follow up question is, how big is the local storage on her device / do you have a computer available.
Many things like contacts, calendars, reminders and so on can be saved locally to the device when you are signing out and then when you sign into a new account you are presented the option to merge data from the phone to the new account but it will take some time to complete the action.
Music and shared purchases can be handled by something called Family Sharing, though that will require a little further explanation too, lets go one step at a time and keep with photos for now.
