I don't like to play the Devil's advocate, but I think a JW could answer your questions.
First, as pointed out, the seven congregations of Revelation really were on their own. If the book of Revelation was written toward the close of the first century, the activities of a governing body would have already been abolished by the Roman destruction of Jerusalem. It can be disputed that there really was a permanent governing body in Jerusalem, but anyone who could have acted as a governing body would (according to tradition) have been driven east of the Jordan River into a mountainous region known as Pella, and the local congregations would have been left on their own. This, according to JW thinking, would have been OK for that time because the Christians were headed toward the apostasy.
Secondly, remember that in theory there is a distinction between the governing body and the FD&S (though not in reality among JWs) - the FD&S is simply the spirit-anointed congregation. So the ones being addressed in the letters were the FD&S of that time.
Third, why weren't they specifically called the "faithful and discreet slave" at that time? Because the question regarding the identity of the slave (Matt. 24:45) was given for those living in the last days, and thus the concept of an FD&S - while referring to a reality that has existed since the first century - is one specifically to be formulated and used in this conclusion of the system of things.
I hope I'm not turning into a Watchtower apologist!
