2) It would be detraction to bring up a sin that someone has committed in the past if the audience that was made aware of that sin moved on. Meaning the audience stopped talking about it, forgot about it (even for a short period), and moved on to bigger and better things. That is the point that defines when it becomes detraction again. Its pretty clear.
That was my understanding of it also. But perhaps I need further instruction.
If the intent of the person(s) resurrecting the past incident - which is now forgotten - for a new audience, is malicious, then of course it is sinful. However, the person(s) may think they have good intent - perhaps justifying their action as a warning, for example.
I ;thought there was a process to avoid this sin? That is, I discuss the situation directly with you to confirm the information that came to attention. If you corroborate it and do not correct it, then I go to the authority (parish priest, forum moderator, etc) for it to be corrected privately, or escalated as needed, in a Catholic manner. We all fall too easily into projection, speculation and then gossip.
Fantastic meditations about the evil of the tongue by our great saints.
In this situation, shouldn't the offended discuss the incident with the person who committed the offense, if possible? Always in an honest, straightforward manner, without childish sarcasm and insults - just man to man. I presume both parties are Catholic.
IF, perhaps the person who is furthering the past incident to a new audience can be approached. Blessed are the peacemakers, all the more so when one has to eat a truckload of humble pie.