"The king, then, is angered with a burning anger against the unmerciful servant, and delivers him to the tormentors -- the evil spirits (for the evil spirits are the real tormentors of mankind). To whom would he who had, from lack of compassion, fallen away from God, and whom God had called evil, otherwise be able to turn -- to whom if not the greatest bringer of evil, the devil? Why does it say: "till he should pay all that was due to him"? In order to show that he was given over to eternal torment.
First and foremost, it is unthinkable that a man with such a debt can ever pay it off; and secondly, because God does not pronounce such a final condemnation on a man in this life, but only after death, when there is no more repentance nor any possibility of paying off the sins committed on earth... If we do not forgive our brother, and we do not do this from our hearts, with compassion and love, then God, the Creator of both us and our brother, will act with us as the king did with the unmerciful servant. We shall be given over to the tormentors, the evil spirits, who will torment us eternally in the kingdom of darkness, where there is ceaseless wailing and gnashing of teeth. Were it not so, would the Lord Jesus have told us? He said this not only in the context of this parable of the unmerciful servant but also on a number of other occasions. "With what judgment ye judge, ye shall be judged; and with what measure ye mete, it shall be measured to you again."
Is this not the same teaching, without ambiguity or reserve? Did the Lord not place this selfsame teaching in the greatest prayer that he gave us, the Lord's Prayer: "And forgive us our trespasses, as we forgive them that trespass against us.""
Now St. Gregory of Nyssa does say this: "...the indebted man was delivered to the tormentors until he should pay the whole debt; and that means nothing else than paying in the coin of torment the inevitable recompense, the recompense, I mean, that consists in taking the share of pain incurred during his lifetime, when he inconsiderately chose mere pleasure, undiluted with its opposite; so that having put off from him all that foreign growth which sin is, and discarded the shame of any debts, he might stand in liberty and fearlessness.
But this seems to be in one of his passages where he speculates on the ultimate destruction of all evil -- "But He that becomes "all" things will be "in all" things too; and herein it appears to me that Scripture teaches the complete annihilation of evil. If, that is, God will be "in all" existing things, evil; plainly, will not then be amongst them; for if any one was to assume that it did exist then, how will the belief that God will be "in all" be kept intact? The excepting of that one thing, evil, mars the comprehensiveness of the term "all." But He that will be "in all" will never be in that which does not exist."
If I'm reading that passage correctly, and I'd like to think I am, then St. Gregory's ideas on this score are tied to his ideas about all being eventually saved -- even Satan and his angels. This is a speculation of St. Gregory that was firmly rejected by the Orthodox Church.
There is no redemption for the fallen angels. The nature of their creation and fall is distinctly different from ours.
I know that there is little to none patristic support for my reading of Matthew 18. I came to this understanding all alone (does it make me Protestant?). I did read subsequently that St. Francis de Sales had offered something similar, but I could not find it in de Sales, and I did not look in any late Latin fathers.
However, the patristic support that you cite does not satisfy. Why is the second condemnation different? Why verse 25 treats the payment as something possible, at least if combined with the proceeds from the sale to slavery? In verse 26, likewise, the debtor treats it as large but not infinite. There is no scriptural warrant to treat the payment as impossible in 34, and to therefore treat the sentence "until he paid off the debt" as some kind of cruel mockery.
Secondly, the patristic interpretation makes the Landlord reneg on his forgiveness, which is inaccurate from any theological standpoint.
Lastly, the parable, like any such parable, anthropomorphizes God, and therefore it should not surprize us if it makes Him an indirect torturer. After all, it does point to the debtor's lack of charity as the real source of the torture, just like the doctrine of Purgatory would teach.
Naturally, the doctrine of Purgatory in no way suggests that all are eventually saved, so whether St. Gregory of Nyssa posited his reading in that unfortunate context or not, his reading remains one patristic authority I will be happy to take.