Is it obligatory to inform and seek forgiveness from those who have been slandered, whether in their presence or absence, and is it obligatory to recant in front of everyone, knowing that they are aware of the slanderer's falsehood and that the intention was to insult, not to slander?

What you have done is a great sin, and slandering chaste women is one of the seven destructive sins. You must repent from it. Among the conditions of repentance is that you deny yourself, unless you fear a greater harm. In that case, you should limit yourself to supplicating and seeking forgiveness for the slandered person, and speaking well of them.