Karma thresholds change over time, due to karma inflation ("karma inflation" = as more people join, the peak scores for stories and comments rapidly increases, since the audience is now much larger). As best as I know, the limits are:
See here for formatting syntax.
Head honcho is pg (Paul Graham).
Yes, you can download some site data, see here.
Yes, moderators can kill articles (called "marking them dead"). Hidden stories can either be dead or deleted. You can view dead comments/stories by turning on the 'showdead' bit in your preferences.
There is a internal message system implemented, but it's only turned on for YC applicants.
Full list of static pages: