2) execute: ispconfig_update_from_svn.sh
3) No. Why shall we dupliacte the effort to writa webmail application as all common webmail packages that support pop3 or imap are compatible with ispconfig.
5) ISPConfig 3 is beta, so there might be problems. But especially the mailserver part is very solid in ispconfoig 3 and there are no open bugs for this part and no changes planned until the final version, I use the mail server as production server for nearly a year now without any problems.