Your 14 year old will be absolutely fine to wander about by herself - its so safe and we have found RCI cruises to be amongst the best for teens making friends - i barely saw my son some days!
If she goes to the teen club on the first day she will make friends and spend most of the evenings there, its a great holiday for independence without the worry! Get her the soda package so she can get herself a drink when needed without you.
I enjoy having the drinks package as i pay it in advance and can just enjoy whatever i want anytime without a big bill at the end.
Book the coliseum for the afternoon, you dont want to worry about rushing on the departure day.
If you have paid gratuities and drinks package we only have extra spend on the ship if we choose to buy anything in the shops - there will be a small shopping mall on the ship with a variety of items in- usually pretty expensive to be honest.
Take a credit card and some euros for the days off ship, thats all you will need if you have already arranged your excursions.
Watch you tube videos of the ship - there will be tours, reviews, guides etc - i find them so helpful in preparing and also helps get excited for the holiday!
You arent in the cabin much and ive done a variety of cabin grades. I do think the biggest you can afford is worth it as 4 people changing at once in a small one is a bit tight- but you can just take turns if needed.
And take any type of medicine you may think you will need, the ship only stocks basic items ( there is a medical centre for more serious illness, but i remember getting bad cold/flu on a cruise once and wishing i had some lemsip with me)
Its an amazing holiday, enjoy!