Rene,
1) Comments in K2 are flat. In that sense, you can't have threaded discussions. If you need that, you can just use something like Disqus or Facebook comments, or e.g. CComment (from
compojoom.com/joomla-extensions/ccomment - free and pro versions available) which integrates with K2 replacing its comments system.
2) If there's an email address already on the system as a Joomla user, then that user will have to login first.
