The "watchdog" function is very easy to do with Dreamhost. Just set up the "junior staff" email address to also automatically forward to a "senior staff" mailbox/address (which does not have to even be on Dreamhost) and everything received by "junior staff" will be also received by the "senior staff" mailbox. You can make the addition of the "forward to" address from the "manage Email Addresses" menu selection on the Control Panel.
The "copy of mail sent" by "junior staff" is another matter, and I can't think of a way off the top of my head to do it conveniently on Dreamhost. Any of you procmail wizards out there know how to do this?
You might be better off not assigning "junior staff" an email address on your server other than a forward to a POP3 account that does not have "webmail" access, and require all outgoing mail use a server or account over which you can enforce administrative management via POP3 at your office end (firewall, special software, etc.). Not much of an answer, that, but I'm just responding off of the top of my head regarding the "outgoing" monitoring.