I vote for changing into office clothes when you get there, at least the shirt. And unless you're terribly uncomfortable, I wouldn't bike in bike shorts, just regular shorts or pants. When I first got my bike shorts I felt terribly self conscious in them. I don't anymore, but I don't wear them for <1 hr rides.

One reason to wear different pants is that biking wears out the seat of the pants. (I guess that could be a reason for wearing bike shorts, since they should have extra reinforcement.) All my slacks & jeans have a shiny butt from biking. It's not glaring but it has been noticed.

I don't worry about parking my bike outside. If it's pretty wet, I try to remember to give it a wipe down and lube when I get home, or if I forget, then the next day when I notice it's rusty I have to clean it and lube it. Except I don't have time so I make a mental note to do it later, and repeat the next morning when I notice it's still rusty.

The advantage to taking care of it before the chain rusts is that the chain will last longer. The advantage to taking care of it before you've been riding around with a rusty chain for a while is that the cassette will last longer. The advantage to taking care of it at all ever is that your shifting will be rough until you get it done. It's not the end of the world if you don't ever do it, it just means you'll need to replace your chain & cassette sooner. When my bike was more sheltered I could get 4500 miles out of a cassette with proper care.

Smart thinking about having the extra underwear etc. That really comes in handy on a rainy day when you discover that you are wet all the way through!

Good luck with the commuting!