I'm not sure about allowing them to be as slobbish as they like in their own rooms.
The room may be theirs, but it's still part of your house.
Children aren't born understanding that if they leave bits of food under their beds, eventually things will grow on them and there will be an unpleasant smell, and the food might even attract vermin.
Similarly they don't automatically know that life is easier, simpler and altogether less stressful, if you don't have to spend hours looking for things because there is 'a place for everything, and everything in its place'.
It was always an uphill struggle for me to be self-disciplined enough to follow my own advice, but I came to realise that an untidy house actually makes me feel quite stressed and not at ease with myself, mainly because I eventually got a cleaner and it felt so wonderful when she'd been, that I was motivated to maintain those standards.
So I don't really agree with leaving the mess for minibags to come home to, as that will reinforce the idea that actually it's normal and ok.
You don't say what you do at breakfast time bags. I always used to set the table the night before, mainly to save time in the mornings, but it sort of set the scene for an orderly day too.
In your position, I would sit with her, make sure she uses a plate (there'll be fewer crumbs that way). You say she would do it if you ask her - so ask her. Teenagers do need reminding on about things (training!) and eventually it will become a habit. What you need to avoid is normalising the bad habit!