If it's a friend you know is using SWC, ask them for it. Better yet, tell them to list you as a multi when they join. If you know that you share a connection with someone, you need to register.
If you just attend a university or use a public connection, as Kendall points out, we can usually tell, but even if it isn't possible to determine, we have only about 2600 active accounts.. out of the total population, it's extremely unlikely it will cause a problem. It never does.
Nope, no extra monitoring occurs. You just get added to a really long and laggy list of handles who declared themselves to be different people and therefore don't get automatically banned for sharing a connection.
That's some strange logic. A bit like refusing to get license tabs for your car because you're scared the people at the licensing department will follow you everywhere you drive. I really hope your friend's paranoia doesn't get you banned and hope you can convince him to follow the rules. I'd help if I could, but with logic like that, I wouldn't even know where to begin.