You can get in without the key, but you do have to join the college. You do the Saarthal quest, and when they send you up to tell the Arch Mage, he is in his quarters. You can stroll right on in. Now, I avoid those gems like the plague, so I have no idea if you will be able to sneak it while he's not looking, but...
Brynjolf ...anyone ever try to kill him? I don't know if it's a glitch or what, but I outright assaulted him in the town square, twice. My fault, my initiative, both times. Surprisingly, the towns people went at him right along with me, though the guards came for me, but he never loses an iota of health...no matter how high a level I am, what I hit him with. He is apparently glitched to be invincible. I am accustomed to not being able to kill certain NPCs, but you can usually force them to take a knee. Can't even make him blink.