1. No, you'll get regular emails of updates concerning your 'contacts' which tends to trigger a trip back onto the site.
2. Personally I'd say you must know them and they must know you, I would never connect with someone 'random' or request connection to someone i'd heard of who wouldn't know me.
3. Not as far as I can see, only in terms of public and private profile and links.
4. Depending on what personal information you place on your public/private profile I would be prudent about the connections you accept, public profile info is accessible by anyone, private (including your own connections) by contacts only.
5. I won't worry about this - linkedin is a business tool empowering access to current and former acquaintances plus groups of like minded or educationally and professionally related individuals, useful for asking/answering questions and in part job or employee hunting and snooping a bit on what people are up to.