As many of us that understand the parable to be just that, a parable, is there a way that a JW can actually question it, even if so is done under the idea that is actually a prophecy?
We are all very familiar with Deuteronomy 18:22 and the way to identify a true prophet. Is there a way you could possibly identify a bad prophet as soon as the prophecy is revealed? Not having to wail until the said fulfillment? Why would you want to do that?
Well simply put, we have come to experience that in the modern world, "prophets" have become pretty sneaky. They lay the prophecy in front of you, just one, may be two of them, and then they spent the rest of their time looking for a reason to kick the can down the road and claim the prophecy still valid. Overlapping generations is one of the first things that come to mind, but I am sure we can find more.
Deuteronomy 18:22 is one of the most regurgitated bible verses used to supposedly identify true prophets. Another paragraphs that we rarely use as JWs (it's an inconvenient one) is the one Deuteronomy 13:1. Let me paste it here for your convenience
If a prophet or a dreamer of dreams arises among you and gives you a sign or a wonder, and the sign or the wonder comes true, concerning which he spoke to you, saying, ‘Let us go after other gods (whom you have not known) and let us serve them,’ you shall not listen to the words of that prophet or that dreamer of dreams; for the LORD your God is testing you to find out if you love the LORD your God with all your heart and with all your soul. You shall follow the LORD your God and fear Him; and you shall keep His commandments, listen to His voice, serve Him, and cling to Him. But that prophet or that dreamer of dreams shall be put to death, because he has counseled rebellion against the LORD your God who brought you from the land of Egypt and redeemed you from the house of slavery, to seduce you from the way in which the LORD your God commanded you to walk. So you shall purge the evil from among you.
Not to spend much time trying to make sense of what it really meant to be "tested" by the Lord (and then have you kill the poor peasant God used for the test) but we can take a few things away from Deut 13:1. One of them is that, contrary to what the WT wants us to believe, we do not have to believe the prophet when his prophecy does not make sense. Even when he has signs and miracles but he directs us to do something that is contrary to God's will, then we must question it.
So how does this apply to the "prophecy" of the FD&S? For starters, JWs did no pronounce the "prophecy", they merely interpreted as a prophecy. Except they made one mistake.
Even if you go as far as considering this a prophecy, it helps to wonder...if the bible is a letter that God wrote to his people in which he details his wishes and rules for a better life, what would he make it so that we could not understand it? I mean, sure the language was different but I would think that such powerful God would find a way to make sure the message remains intact.
If we can not understand the letter without the FD&S, why is there so much room left to identify said slave? If Moises was the only one who could communicate with God and get his word, what would happened if in a flash of "new light" the Israelites would have concluded that Moises wasn't the one sent forth after all? In other words, how can ever know who the true F&DS is if his identity is within the letter that we can not understand without the FD&S? Furthermore, if the identity of the slave is open to change, what is to be said about the "truths" he fed us in the past? How can we be at the edge of the great tribulation without having firmly identified the one who is supposed to provide the guide from God? Should have not he been identified first and then began to serve spiritual food?
You see, this idea of the slave providing food at the proper time, even though the identity of the slave could change is not compatible with God tradition of giving accurate forewarning and information. Unless I am missing something, God would have not driven the Israelites to the edge of the Red Sea and then say... here is your leader, listen to him... oh wait no, is that other guy over there... oh may be it is both of them as a "class". You get the picture, just do whatever they say but hurry because the Egyptians are on your heels... bye!