So, if I'm getting this right: she's 15 now and she's going on this trip, but not the next one, and you're trying to prepare her (and yourself) for that time?
I'm just going to assume that you won't leave her alone if she's not ready. I would start working on the idea that she lives in "your" home, that she doesn't own many of the things in it, and therefore you get to set the rules about how your home and things are used. Whether you are there or not. This perspective will give her less feeling of entitlement to having parties, etc.
Second, when you do go away, you can help her plan something that you do approve of, something that will be fun for her eg. shopping and dinner with an older cousin, friend, etc. And make sure family members know that she will be alone, so they can "forget you are away" and call the house. They can also remind her to call them if she needs anything. On the same note, I would let her start helping you with some of the duties you do as a homeowner eg. could you go and check the oil to make sure we have some?, could you call the plumber and set up a time to have xxx fixed? It would be really scary if something went wrong while you were away and she had never called a plumber before.
Third, and most important, is bribery. If she does as you ask while you are away, you can give her money towards a class trip or a shopping trip or something. I got the class trip the first time I was left home alone.