In the Nineveh case the main message is that God offers forgiveness, but logically this also means there is something to forgive. And in this case it’s a temporal punishment. That’s a simply and easy deduction to make.
Except that it’s the wrong deduction, since it doesn’t take into account the primary intended audience and how they would have understood the story.
“Temporal punishment” is all that the intended audience would have understood as how God punishes (or rewards, for that matter). There was no understanding at that time of “eternal bliss in heaven” or “eternal punishment in hell.” So, to “deduce” that “temporal punishment in the story implies the same thing that ‘the temporal punishment due to sin’ that the Church talks about” is
explicitly a false conclusion.
And either way, at some point it would become clear to everyone that God wouldn’t punish the Ninevites, so so time period after that could be established with their deaths being unrelated to God’s punishment for prior bad actions.
OK, fair enough: their (later) deaths would be interpreted as “God’s punishment for (later) bad actions.” Not seeing how this proves your case, though. Again: they had no idea about heaven, so the notion of a distinction between “eternal punishment” and “temporal punishment” just wouldn’t make sense in the context of this account.
Regarding eternal punishment - well the point is that if they died later it is unlikely the Ninevites would be punished with temporal punishment for their previously repented of sins. They wouldn’t have to continue their punishment for previous sins in the afterlife because God cancelled the punishment due to their repentance.
More bad reasoning, I’m afraid. Look: the original audience for this account would have interpreted the “cancelling” as the “cancelling of God’s punishment”, without the nuance of “eternal” or “temporal”. Therefore, it’s illogical to suggest that
we can impose a different framework on the narrative, without doing damage to the lessons it’s attempting to teach.
At best, we can look at the story and say, “well, their immediate punishment was taken away by God; if we understand this action in light of later understandings of God, then we might say that there was ‘forgiveness’ of a sort (although… there really
wasn’t forgiveness, properly speaking – St Paul mentions that not even the prescriptions of the Mosaic Law brought forgiveness!).”
We might also note that the whole “sackcloth / ashes / fasting” is
itself penance, which is something that we continue to do in order to provide satisfaction for our sins. However, in the same way that the “three Hail Marys” that we say after confession don’t necessarily remove
all the “temporal punishment due to sin”, neither would it follow that the Ninevites’ actions completely wiped out
their requirement for reparation, either.