Where can I find the key to the top roof of the College of Winterhold?
Ben,
You will get the key when you become Arch-Mage of the college.
I am not sure where you are in the quest line, so don't know if you need Mirabelle to progress in the quest line. I hope not.
Part of the quest line to become an Arch-Mage does require talking to Mirabelle. See: http://supremegamer.hubpages.com/hub/skyrim-defeating-morokei.