Salam all
My contention was not whether hell is eternal or not. I do not know for sure. I just pointed out that that the afterlife is eternal so both bliss and punishment maybe eternal. That's all. Infinite afterlife's fate is dependent on this finite life. What will happen is not only subject to Allah's mercy but His justice too which is perfect.
When reward in heaven is discussed, sometimes it is said that one is gonna get multiple times that what he or she deserves. That is Allah's decision and I believe when He says that sin is counted one time He is alluding to His mercy, not that the punishment is lessened. Reward is multiplied punishment is not. But punishment in some cases is extended, now this extension may mean increase in intensity of punishment, new more painful punishment maybe introduced as well, lengthening the duration of punishment is not the only way a punishment can be extended for a sinner. Everything is not so simple as it seems.
019.079
"Nay, but We shall record that which he says and We will extend / prolong (* yamuddu) for him from a span of punishment extensively"
But this does not mean that duration of punishment is lessened, maybe intensity of punishment is less for people who have committed less grave sins. Anyways ranks exist also in hell, so maybe they will occupy those ranks where punishment is less severe. Allah has said not an atoms weight of injustice will occur and each will receive what they deserve. I find solace in this. For me it doesn't matter whether hell is eternal or not, I will not be wronged that is what reassures me.
Another thing that I refuse to do is argue about the hereafter with certainty, because Allah talks about the hereafter using similitudes, and this much is certain nobody knows what is reseved for us there in terms of both punishment and rewards. Hell's punishment is severe so is the bliss in heaven. Intensity and rank is what makes the difference maybe. Duration may not be of importance because time as an entity may not exist the way we understand it on Earth.