1. Yes. No one can hack your wallet just by knowing your bitcoin address. The only reason to not publicize it is due to privacy reasons.
2. Not sure.
3. Activity refers to how active you are here on bitcointalk. If you post at least once per day, you can gain the maximum activity (14 activity every 2 weeks). For more info about this, read around the forums. Can't link you to threads since I'm on mobile.
To fill in the blanks
Answer 2:
https://bitcointalk.org/index.php?action=draftsAnswer 1 and 3 were already answered by mjglqw