You'll be surprised at how soon you'll be able to bike 12 miles, especially if its a reasonably flat 12 miles. If there are hills, just take them slow.
Did you drop your chain while shifting in the front or in the back? Rollin is right, that shouldn't really happen too often -- I only drop my chain when I do something dumb, like shift in the back while the front is still coming down, or shift in the front when I'm already putting a lot of pressure on the pedals. Take it to a bike shop and see what they say. Were there any other issues with shifting? (Stickiness, jumping, etc?)
Re: the buzzing, do not hesitate to take the lane. Once you get more experienced, you'll learn how to control the traffic behind you. Iit is much easier that you would expect. Drivers are very responsive to hand signals, if you tell them to stay back and when to pass. On a busy 2 or 4 lane road, I would take a whole lane to force drivers to pass, especially if the lane is narrow. Buzzing is scary, and the only times I've come close to an accident were when I failed to take the lane and force drivers around. FYI, most car-bike collisions happen in front of the cyclist (getting T-boned, left turning cars, etc) -- its extremely unlikely anyone will hit you from behind. This was my initial fear about taking the lane but now I believe its much safer. Especially when a line of cars is passing you while sharing the lane -- the lead car tends to give plenty of space, but they tend to get incrementally closer, and a car 3 or 4 back might not even see you.
Any attempt at bike commuting counts as badassity - I'm guessing most of your coworkers would be amazed and horrified that you were even trying it :)