Journeyman,
First, you don't get to exclude secondary lessons of Jesus' parable of the 2 servants simply because the primary lesson is different. One secondary lesson is that the unforgiving servant is neither executed nor given a life sentence; rather he is consigned to debtor's prison "until he should pay back all he owed (Matthew 18:34)." This place of torture is an image of Gehenna and the implication is that the debt can theoretically be paid. Your claim that he can't ignores Jesus' routine use of Semitic hyperbole and is in any case not stated in the text.
Second, Paul's use of "the day" in 1 Corinthians 3:13 leaves unclear whether he has in mind a preliminary judgement at death or the final judgment. But even if you are corect, that does not mean that the believers in question can't be retrieved from Gehenna and be "saved, yet so as by fire." Indeed, the rabbinic use of this expression points in that direction and Paul, as an ex-Pharisee can be presumed to use the same language that he once used prior to his conversion.
My friend, I don't believe in torment until infinity, but there is no doubt the unforgiving servant would die in that prison. This is why Jesus said,
Be ye therefore merciful, as your Father also is merciful. Judge not, and ye shall not be judged: condemn not, and ye shall not be condemned: forgive, and ye shall be forgiven. Lk.6:36-37
But there's no reason the unforgiving servant could not have a change of heart in prison and forgive his fellow servant, instead of trying to collect for a debt (millions of dollars) that could never be repaid. God is merciful to the repentant. If we desire God's mercy, we should desire it for others.
Even in the "the rich man and Lazarus" story, Jesus wasn't advocating torment without end. Abraham tells the rich man,
Son, remember that thou in thy lifetime Lk.16:25
And this is what the judgment seat of Christ is, an examination, an illumination of lives lived. This is why "torment forever and ever" refers to exposure of unforgiven sins (from "eternity past" if you will, all the sins of the unsaved from beginning to end). Satan himself will be turned to ash.