1) You cannot edit your post anymore after 24 hours. (If it's really important you could ask Mod+ to edit it for you)
Now I mention you, you get a Notification. We use it to alert someone at certain Threads etc
3) Credits & Levels don't serve any purpose (yet).
4) Activity is based on the frequency of posting.
5) Stickies are used for important Threads, or Threads that should stay at the top of the page for some time instead of reaching page 6 after a few days.
Requirements: Decided by Staff, either really helpful or actual.
^ I hope this answers your questions