Prompt for password instead of requiring it as a command-line argument.  This way, it is neither displayed while you type it nor added to your shell history.  The only argument you should now pass on the command line is <your-itunesconnect-login>.
Prefix the file names with "NNN - " where NNN is the session number.  To avoid copy-paste errors I created a variable ${dest_path} which contains the path each file will have when a download is successful.