One thing you can try and do is to say the opposite of what was said. If one says something evil in public they can cancel it out by saying good in public. If you can make people aware that it is the same personality/online id then you can make it clear publicly that you don't go for that anymore. If that is not possible or feasible then just spreading the opposite message elsewhere can absolve someone from blame.
Walaikum Assalaam
From my knowledge, each sin is counted as one sin. It does not multiply even if it affects others, but sins are of varying degrees because they can affect/harm others or cause mischief in the land, which is why we have to think before we act.
It is only good deeds that multiply. So if you taught someone how to pray for example, each time they pray you get a reward for it. This is not the same with sins. This is part of Allah's mercy.
I'm afraid that's not correct sister.
Abu Huraira reported: The Messenger of Allah, peace and blessings be upon him, said,
“Whoever calls to guidance will have a reward similar to those who follow him, without detracting from their rewards at all. Whoever calls to misguidance will have sin upon him similar to those who follow him, without detracting from their sins at all.” (Muslim)
However, if one truly repents from their sin (even if the sin involved misguiding others) than Allah will, inshaAllah, forgive them and they will not be responsible for what the others do as they are still responsible for their own actions.
There are actually practical examples of this. In the time of the Prophet (Salalahu 'alihi wa salaam). The leaders of Quraish were seeking to kill the Prophet (Salalahu 'Alaihi wa salaam) and did in fact succeed in killing quite a few of his companions (RadiaAllahu 'Anhu). Because of these leaders, many people died in this extreme state of sin and will no doubt be in Hellfire forever. Yet, after the conquest of Mecca, the Prophet (Salalahu 'Alaihi wa Salaam) forgave them and they became Muslim in the end which we know erases all sins. These men died upon Islam as noble Sahabah (RadiaAllahu 'Anhum) even though their previous followers will be in Hellfire forever.
Try to absolve yourself as much as you can. Try to do actions or statements that are opposite to what you did so you can balance out whatever it is you said or did. Try to guide others in a similar way in which others may have been misguided and you won't have to worry inshaAllah. Making Dua' that Allah protects anyone from being harmed or misguided by what was said or done is indeed a very good thing to do. That's a very good piece of advice.
May Allah Forgive us and Make us a good influence on others not a bad influence. Ameen.