I feel sorry for you OP, it must be so stressful knowing a space in your house is like that.
If I were you, I would be saying food upstairs must stop, because she doesn't clean up after herself. This is the problem, rather than its being an inappropriate place for food; it wouldn't matter if she ate a meal up there while studying, it's the failure to tidy up afterwards that's the real issue. So if she won't take the time to take dishes and leftovers, drinks cans and glasses back downstairs, she shouldn't eat up there.
I don't think any of my siblings had a messy room like that when we were teens - we didn't have a lot of stuff to start with, we were very badly off although the house wasn't small. My brothers used to share a room and when it got untidy and our mum complained about it, both boys would say "It's not me, it's him!" Once they got their own rooms, mum discovered one of them had been telling the truth, the other hadn't... 
I do think that level of mess indicates a lack of respect and consideration. Who changes sanitary protection out of the bathroom?! It's the appropriate place, hygiene wise. What if you, er, dripped or something? Bathrooms already have wipeable surfaces and products to clean up, it makes sense to do this in there...
(If I need DS to do something he doesn't want to, he is asked - then threats to block technology start. All technology either has a password he doesn't know, or we as parents have the ability to lock his devices from our phones. I wouldn't hesitate to turn off the internet if I had to, they need to learn that like it or not, some things just have to be done.)