1) You dont have a second server at the moment as you just added a record manually to the databse without any connection to a real server. You will have to delete any records that you assigned to this server in ispconfig.
2) Yes. You always need a dns record for a website.
3) The nameservcers must be exsiting and working nameservers that are used to resolve external domain names.
4) This error is there because you did not add the server correctly. See my other post.
there are no other manuals for ispconfig 3 available yet and as I suggested to you above you should start with a single server setup wheny you are not familar with linux administration.