Questions:
1. If I remember right when you start a campaign and pick which platform to play that is the one you are on until the end of the game then start in a different sub.
2. Thats the main ones, just ask around in the forum for any independent ones out there.
3. Go to
http://www.subsim.com/ssr/vfleets.html or try out
http://www.sealordsvf.org/ for some leagues.
4. If you go to the download sections there may be one online for it. Most of the stuff I learned was just from playing or discussion then trying it myself.