I find it very difficult to link this unmerciful servant parable (Matt 18) with that of salvation?
And that does not surprise me. Somewhere along the line salvation became so utterly of God in the church that we forgot that the free gift of justification is secured through the forgiveness of one's sins for the asking, but instead through this strange but amazing gift that suddenly appeared on our front door step one day and there's no way to not accept it, or return it. You've been picked to receive it completely and utterly apart from any input from you whatsoever and now your stuck with it.
But anyway, the parable is about forgiveness. Justification and salvation are contingent on forgiveness--God forgiving you, and you not showing contempt or indifference and lack of trust and appreciation in that forgiveness by failing to offer that same forgiveness to others. So it most certainly is a parable about salvation.
This man was (present tense) a servant of the King.
Everyone in this present earthly kingdom of God is a servant of the King. But many simply do not live up to the responsibility that comes with having your sins forgiven for free and apart from the merit of your own goodness to secure it.
Do you suggest that we get saved and then have to settle our accounts with our Saviour King...
The Bible is very clear that EVERYONE will give an account of himself to God for what he did, whether good or bad, at the Judgment.
...and if we cannot settle the account, we (and our family) are then in danger of being sold?
Remember, it's a parable. He's speaking in the vernacular of the day. Why you had a DEBT YOU COULDN'T PAY your family was seized to help pay the debt. That's the point--the debt you owe is one that you simply can not pay yourself. That's how the debt of sin is for us. It's simply too big for us to pay ourselves. The literal elements of the parable illustrate this fact.
Do you believe that this debt (for the purpose of salvation) can ever be repaid as Jesus said?
Yes. See what I just wrote above.
edit: Sorry, my vision is failing. I saw 'never be paid', lol. So, 'no', the debt can never be paid. It has to be
forgiven, or else it remains to be paid in full.
These torturers, who do you think they me be?
Again, remember, this is a parable. I don't think there are literal torturers in the place of torment. The point is, the place where those who can't pay their own debt of sin, and forfeit God's gracious gift of payment of their sins for them through their contempt and/or indifference to that gift, will go to a tortorous place where there will be the 'weeping and gnashing of teeth'. And since the debt you owe is bigger than what you can pay--but which must be paid before you can get out--you will never leave that place of suffering. It is a final judgment.
Perhaps the following teaching gives us a bit more insight into the expected duties and responsibilities that receiving God's gracious free gift of forgiveness, and then becoming a servant of the King has attached to it, and the result of not acting on those duties and responsibilities:
"35 "Be dressed in readiness, and keep your lamps lit. 36 "Be like men who are waiting for their master when he returns from the wedding feast, so that they may immediately open the door to him when he comes and knocks. 37 "Blessed are those slaves whom the master will find on the alert when he comes; truly I say to you, that he will gird himself to serve, and have them recline at the table, and will come up and wait on them. 38 "Whether he comes in the second watch, or even in the third, and finds them so, blessed are those slaves. 39 "But be sure of this, that if the head of the house had known at what hour the thief was coming, he would not have allowed his house to be broken into. 40 "You too, be ready; for the Son of Man is coming at an hour that you do not expect." 41 Peter said, "Lord, are You addressing this parable to us, or to everyone else as well?"42 And the Lord said, "Who then is the faithful and sensible steward, whom his master will put in charge of his servants, to give them their rations at the proper time? 43 "Blessed is that slave whom his master finds so doing when he comes. 44 "Truly I say to you that he will put him in charge of all his possessions. 45 "But if that slave says in his heart, 'My master will be a long time in coming,' and begins to beat the slaves , both men and women, and to eat and drink and get drunk; 46 the master of that slave will come on a day when he does not expect him and at an hour he does not know, and will cut him in pieces, and assign him a place with the unbelievers. " (Luke 12:35-46 NAS)
Many probably see this as a works salvation and, therefore, twist it's meaning to divorce it from that understanding. But it is a works salvation only in the sense that the free gift of justification (having your sin debt removed for free) does have an expected and obligatory outcome attached to it.
We are expected to give the goods of the Master's household that have been placed in our care--because we have now been made servants in his household--out to our fellow servants (think forgiveness, mercy, long suffering, helps, etc., but especially forgiveness). The day is coming when we will give an account of the stewardship of these things that have been entrusted to us. And if we have been found abusing others instead of feeding and caring for them with these things that we have been given charge over we will lose our place of service in the kingdom of God and assigned a place with the unbelievers (not spanked and sent on into the kingdom with a bruised bottom as some insist).
The parable of the shrewd manager (Luke 16:1-12 NAS) is another teaching of how we are given the responsibility to forgive others the debt of sin they owe the Master we work for in order for we ourselves to be commended by the Master and have the assurance of a secure future when we exit the abode of these bodies we live in now. 
