1) You can either use mysql replication or you bind the mydns daemon on the slave to the master database. ISPConfig 3.0.2 will also have a special replication function for that.
2) awstats is not supported in ispconfig 3, so you might have to setup it manually.
3) You have to select "localhost" as mysql server on ns1 and ns2. But on ns2, select expert installation mode which will aks you if you want to join a master server and then asks for the root login details on the master server. Make sure that you created a root user in the mysql db on the master server that has permissions to allow access from the hostname and ip of ns2 before you install ispconfig on ns2.
finally i wondering if i could setup 2 servers with all service (dns,web,mysql,mail,ftp,quota and stats) to work togheter. if ns1 gones down work ns2 and vice versa. and all mail and all changes of database to replicate between servers in real time.
Thats not possible, ispconfig manages multiple servers from one controlpanel but what you want is mirroring for redundancy which is something different.