The answer is still the same then in #2. The setup is fine.
SFTP is not handled by the SSH daemon and not the ftp daemon, so you will have to create ssh users to use it which will not improve security as these users wiuld get shell access then instead of having just virtual FTP users. So in general its better to use ftps (which is FTP over ssl) and not SFTP. See ISPConfig FAQ for instructions how to enable ssl encryption for pure-ftpd.