can a roommate break a brand new microwave oven?!
Because of radiation microwave has a safety feature to stop magnetron when oven door is opened. This in itself will not damage oven but to cook afterwards it must be cleared and reset or it will only finish the count down. You will not have as much cleaning to do if you use a plastic dome splatter cover. The greatest chance for damage is running it when it is empty or leaving a metal spoon in a cup etc.
I don't think opening the door before the time is up or not clearing it will matter whatsoever to the use of the microwave. I do think not cleaning it will though - it is not sanitary and will stain the walls.
You should remove it and let her live without one.
If this is a COLLEGE room mate, I am utterly amazed that any college would even accept you! Ever heard of punctuation??