1. You can actually wear your jersey under your wetsuit, if you like. The only reason why you wouldn't want to do that is if it's going to be chilly on the bike and the cold, wet jersey would make you even colder (and possibly slower) than a dry jersey that you put on in transition.

2. For 12 miles, all you probably need is the aerobottle. That should take you 35-50 minutes most likely, and 24 oz of fluid in that time frame is plenty Also, with regard to carrying spare stuff, plenty of people do that (I'd say usually over half). It's really up to you, though I recommend that you carry spare stuff, seeing as how flatting and DNFing is usually far worse than flatting and taking 5 minutes to change the tire!