Do I have to steal the Whispering Door Key off of either Farengar Secret-Fire or Jarl Balgruuf the Greater? Or else kill them? I don't have the pickpocketing skills to steal the key unnoticed, and I don't want to kill them. Are there any other options?

Get a fortify sneak potion if you svck at sneaking, coz I had to nick the key to get anywhere with this quest. Try the old crouch,nick,uncrouch only takes 2 ticks.

Rather than ruin anything for you I'll just say that the door is sealed by the will of a being more powerful than an Aedra (the divines) or Daedra Princes.

EDIT: Nevermind, haha I imediately thought of a different door that whispers up near Dawnstar.

Yeah you can pickpocket the key or kill them. Invisibility potion and pickpocket potion will help, as will items of pickpocketing.

So it's a sneak potion and not a pickpocketing potion?

Or you could place a bucket over one of their heads. Try it with Farengar Secret-Fire when he is in the room with Enchantment table.

...

Not sure if serious...

It requires almost no pickpocket skill. I've pickpocketed it with 3 different characters, absolutely no skillups beyond what they started with.

I relished killing farengar. oh man that was just great. the number of times he told me to visist the college I was archmage of....all that hate channeled into a glorious moment.

The jarl will give it to you I think, since you have done so much to help him in the past, assuming your speech is passable. otherwise fortify sneak/pickpocket potions and DON"T WEAR ARMOR should do a pretty good job right off the bat.

http://www.youtube.com/watch?nomobile=1&v=bmI799kEOSk

Either will help, the pickpocket potion far more so, both together is better.